Possible Citicar upgrade?
 

Hacker Brushless A200-8 - Aero Model Inc.
With this:
HobbyKing R/C Hobby Store : Turnigy Fatboy 300A ESC 5~15S - (OPTO)
110kv*48(if you do 48volts)=5280rpm
Of I remember right, stock is 3200rpm which gives around 30+ mph
5280/3200=1.65
1.65*30+=49.5+mph
So 50mph with 15kw or 20hp peak in a car that weighs about 1/2 of a ton...and in comparison to the stock 3.5hp, you're really having some fun...

I have converted a Razor E300 to have a brushless setup. Stock it didn't really move, and was obviously pointless. Now:
-Turnigy Rotomax 1.60
-9s (37.8 volt full charge, 33.3 volt nominal) 12ah lipos
-Range around 10± miles
When geared up I hit 65+, now its around 35mph top speed. Stock was supposedly 15, but realistically 7-8mph was peak.

Also yes that motor is extremely expensive, and there are cheaper options (which is probably what I would go with).

I'm running the stock motor in my Commuti-car, same motor as the Citi-car and the 3.5hp motor is rated at 6hp when forced cooled on the commuti-car, seeing a constant 200 to 250 amps is common, seeing 300 to 400 amps is common when going up hills and I've seen my motor draw a peek of over 500 amps when going up steep hills, that is why I installed a 650 amp solid state speed controller, so the speed controller wouldn't be the weakest point, instead the batteries with their max discharge rate of around 550 amps become the limiting factor and if your car has the contactor speed controls instead of a solid state speed controller you can see 600+ amps being pulled by this 125 amp motor you have for a few seconds at a time, but it's not a good idea because you can brake or melt parts of the drive train!
I'm sure it's slight over kill, but I did a few upgrades on my car including 00 welding cable between all of the batteries and the speed controller, stock battery cables are 4 gauge, that controller is bragging about having 10 gauge (large the number the smaller the cable size, 00 is nice and big) the old 4 gauge battery cables would get warm, I've melted the posts off batteries, vaporized 8 and 10 gauge wires inside the motor twice now and had my old battery cables to hot enough that I was worried about melting the insulation on them.
Your 3.5hp motor will put out more power at it's peek, around 28hp or more, so don't underestimate it, if you want better performance, save up for some good lithium batteries, but make sure they can handle the discharge or you will destroy some good lithium batteries, if you do end up with lithium batteries you might look at going with 72v and just limit your driving speed so you don't over speed your motor and blow it up.

Well long before a lithium upgrade, I think that an upgrade to a solid state controller is needed. The reason the Fatboy has dual 10 guage is it's lighter than significantly larger wires, and being that it is designed to make a 50+ pound thing fly 100+mph vertical, weight is an issue.
My scooter pulls a peak of around 3700 watts accelerating up the steepest hill around and that is well....fun lol, the wires is 10 guage, and yes, it does get warm doing that. 36+ volts@3700+ watts=100+ amps going through 10 gauge wire and connectors rated at 80 amps.
Unrelated, while also related:
1) What would you suggest for a solid state controller? (Don't have lots of money, but want it to be reliable.)
2) What would you suggest for a lithium pack/balancer/charger when going that route?
3) If you had a lithium pack, what would you do to incorporate solar charging? There are 48v lead acid solar chargers, but I don't know of any 72v solar chargers especially not for lithium. What solar panel? 6 (or 3) small 12 (or 24) volt ones or are there any 72 volt ones?
4) How many amp hour (ah) is the 48v pack in the citicar (supposed) to be to provide the supposed 50 mile range which no one will realistically hit with lead acid?

If I'm going 30mph on flat ground my car pulls 6,000 watts, that is after it is up to speed with no head wind, a small incline in the road will make it jump up to 8,000 or 10,000 watts, the steepest hill I've climbed in my car I ended up pulling over and slowing down because the gauge was reading over 30,000 watts and I didn't want to melt or burn up anything.

I have an Alltrax controller, I bought their AXE 4865 controller because I have hills around here and I wanted to over size the controller so it would be bullet proof, other people I've talked to have used the AXE 7245 controller, a 450amp peek controller, but the only real advantage that I see to that one is that you could upgrade to a higher voltage battery pack while keeping that speed controller, buying the kit to build an Open Revolt speed controller is going to cost a little less then buying a fully built, warrantied, ready to install Alltrax, but if you have more time then money and want to build a speed controller it might be worth while.
Do NOT buy a Kelly controller unless you like stuff that starts on fire and a warranty that is worthless.
Other options for 48v speed controllers are pretty limited, GE makes some 500 amp golf cart speed controllers that you can get from golf cart shops but they are about the same price as an Alltrax and the AXE can be programmed and has other nice features, Curtis also make some nice speed controllers including some 48v ones for golf carts and fork lifts, so it's worth checking around to see what you can get used as well, most of the better built speed controllers will tapper back your throttle instead of burning up, but the more you push the limits of the speed controller, like say running a 350 amp speed controller in your car, the shorter it's life span is going to be because it's running at closer to it's peek nearly all the time and it's going to be hotter as well because of this so it's going to wear out sooner.

Alltrax AXE Products Page

Stuff with Lithium is changing all the time so doing research in a year or two or whenever you can do lithium might be the way to go, people I know like the GBS lithium batteries and a few people I've talked to have ThunderSky lithium batteries, some people claim you don't need a BMS but if you skip it you run the risk of destroying the battery pack, a cheap BMS that is just an over charge protection with a shunt resister can have issues as well, have heard of people having them start on fire due to over heating, a good BMS needs to be able to communicate between batteries as well as shut down the charger if a cell is getting to full on top of shunting power past the batteries that are full.
But as I said, when it gets closer to getting lithium batteries do your research then as you will have a ton more options then you do right now, although you do have a bunch of options right now.

First off, solar panels don't really belong on a vehicle for many reasons, but you can get battery chargers for lithium batteries and you can get 72v charge controller s for solar panels for lead acid batteries, but they cost $1,000+ last I checked.
My dad was asking me more or less the same question the other day, wanting to know if he should rig up a system so he can charge their EV right off their solar panels and what we came up with was it was going to be best to have the solar panels tied in to the house power grid and we settled on that because then the solar panels are always doing work and when you plug the car in to charge it you are using the same battery charger as you would any other time and you aren't having redundant battery chargers on the vehicle.

battery pack amp hours will vary a little, buy batteries from Wal-Mart and you might only get 180ah (20 hour rating) batteries, while I see the Trojan T105 have a 225ah rating (185ah 5 hour rating), so in the same size batteries you can get a range of capacities, the higher capacity is why I went with the Trojan's, that and they have a great reputation, I'm not sure who claimed you could get a 50 mile range out of a Citi-car, all the sales stuff I've read from the 1970's said 40 miles, my batteries are 3 years old and driving 18 miles took them down to 52%, taking them down to 20% is said to be ok by some people but it will shorten your battery life! so I might have been able to get 40 miles on my pack when it was brand new and at 3 years old (I take good care of my battery pack) I'm sure I could get 30 miles out of them and still be fine but I wouldn't do that kind of trip more then a few times a year and I would want to charge them up as soon as possible! so usable AH in my pack I figure is around 100 amp hours in a 225ah pack, that gives me a nice safe buffer to keep my batteries happy and because of that I figure I'm going to get 5-6 years out of them while some people figure they are replacing batteries ever 3-4 years, but for me to drive to work my pack is at 73% full according to my PakTrakr gauge and 85% full according to battery voltage after sitting for 3 minutes, at that point I plug in and recharge at work, so with shallow discharges like that my batteries are staying really happy, I still put a lot of miles on them but I charge them whenever I can too.
If you are looking at lithium batteries, 100 amp hour cells are about as small as I'd go with unless you bump the voltage up, lower amp hour cells are not going to handle the discharge rates that you might see going up hills and so on.

Keep in mind, you are driving a registered, insured car, I am talking about a scooter with a motor the size of a fist pulling 3700 watts. I found a used alltrax 7245 for sale $365; what are they new?
Btw I did mean 40 mile range, I am typing on a 4.0" screen, and obviously, autocorrect doesn't change your numbers. Also thanks for such detailed answers.

Around $550-$680, if it hasn't been used a whole lot then it sounds like a good deal, you might find one a little cheaper from someone who tried putting it in a larger car and found it to be undersized, but that is such a nice size speed controller for our cars and for golf carts that they seem to hold their value.

And a 4844 for $175

So I am between an Alltrax 4844 for $175, and an Alltrax 7245 for $365, more than double the cost, but the same amperage...the possibility to go up in voltage sounds great to me, but, I am a 15 year old with not too much money, and I still need another "real" car, so the extra $200 might have to wait for another day...

The 7245 is rated for 450 amps for 2 minutes, while the 4844 is only rated for 400 amps for two minutes, where you will notice it is once you get past the 300amp 5 minute rating of the 4844 is that it's only rated for a sustained 1 hour rating of 150 amps while the 7245 can sustain 200 amps for an hour, that extra 50 amps in that 5 minute to 60 minute time frame is a 25% drop in power, with my 4865 speed controller I've felt it drop from it's 400amp output to 250 amp output, so I'm not sure what having it drop down to 150 amp output would be like but it would make going up hills a rather slow event.

Alltrax AXE Products Page