I totally agree with the extra battery packs, you could even fabricate a bracket to hold cordless tool batteries, then he's have faster charging lithium batteries with a bit more zip to them.
But to answer your question, a 50cc gas engine and a single car alternator should be plenty! specs say it's two 12v 8amp hour batteries and that they can run for 45minutes at a time, so it's drawing around 10 amps... a bit more then those batteries are really designed for so a 2nd pack on the go cart is going to more then double his run time.
Anyway, you don't need two car alternators, you just need one and you need to either adjust the voltage regulator or remove it all together because you can regulate the voltage by spinning the alternator the correct speed for that voltage or in a car it would otherwise be to high so it has a regulator on it, also your alternator can put out up to 50 amps (at a high enough speed) so putting out 10 amps shouldn't be a problem at all, it might even be over kill, 10 amps times 24v is 240 watts or about 1/3hp, losses of course in converting mechanical to electrical, but even at that a 1hp gas engine should be able to put along and keep that go car running for days on a single tank of fuel.
How Do I Charge a 24V From a 12V Alternator? | eHow.com
How to Adjust a 12-Volt Voltage Regulator | eHow.com
should give you some ideas on how to adjust what you have.
Off the top of my head I can't think of an easy way for it to automaticly start up the gas engine when the voltage gets to low, unless you already have an electric starter on the 50cc engine... but I doubt it and alternators make lousy motors... you pretty much end up building a motor from scratch.