Somehow I missed this thread....anyhow I'm jumping in to put in my two cents without reading anything but the first page.
I was into the mini bike scene for several years so I got some input for the power plant. Stock Honda 125cc horizontal motor will only put out 6-7 hp, vertical motor is about the same. I built up a chinese clone with 12.5:1 CR, big cam, carb and pipe and it dyno'd around 11hp. Cost me about $200 to build, not including cost of motor. The max I ever saw out of these types of motors was 23hp with about 10K invested in a custom DOHC Tak. setup. 17hp is possible with about 3K invested, and this would be a dedicated race motor with very little hours before needing completely rebuilt, oh yea, race gas only, so not to Eco freindly running the 110LL
I would lean (pun intended) towards the Honda based Horizontal motors because it would be much easier to duct air to it for cooling vs. the vertical motors, considering a full fairing would be used.
The chinese motor route would be very appealing because you can buy up-sized "hybrid" Honda/Kawasaki motors up to 160cc and making a reliable 14-16hp, which put's you in a range that makes powering this vehicle possible. Price for this motor will be in the $400-500 range and it would bolt up to any Honda horizontal frame, which are plentiful and easy to register. The nice part about the chinese route is cheap and plentiful parts. If you've ever been to a Honda bike shop to buy parts you will know what I mean...
I've had some ideas rattling around in my head for a while now... I got an old Trail 90 sitting in my garage, the dual range transmission variety. It would make for a good streamliner. You can make the seating possition very low and they get 90mpg with the throttle pinned. It might be reasonable to see a full faired version of that able to top 200mpg! That would be a fun project, just need time.....and a lot of fiberglass
Oh yea, the Trail 90 will only touch 40mph with a full tuck, downhill, with a tailwind, maybe 5hp? My hoped up, 125cc, 11hp, race bike will wheelie past it at that speed