my concern is the alt not working below (lets say) 100 rpms... / needing to be at top speed to get anything out of it (even if i pulley it (the axle) as large as possible)...
I found this in case you didn't already: http://northwestkarts.com/files/CF150ServiceManual.pdf
The amount of vague specs on this thing are amazing. Some random mentions on google say it will go 27mph top speed. Assuming that is achieved with 22" diameter tires (seems to be a popular rear tire size on buggies), that gives you around 400-RPM rear axle speed at the top end.
So in order to drive an alternator, you would need somewhere around a 10:1 pulley ratio for the alternator to be useful at anything other then full speed. Assuming you have a 1" alternator pulley (which is already pretty damn small), you'd need a 10" pulley on the drive shaft for it to spin fast enough. At 1" you'd have to use a ribbed belt not a v-belt too. V-belts don't like small diameters.
It would be better if you could drive it from the output shaft/pulley/variator (whatever it has) but that looks like it's sealed with a cover. That just leaves trying to drive it from the stator fan and I doubt that's strong enough to support the tension of a belt/pulley. Pop the fan shroud off the thing and see. It very well might be beefy enough to support 2-HP or so considering the magneto spins from it. If that's the case then it's just a matter of rigging a pulley to it.
To make things easier use a mini-alternator. They range from 35-50 amps and are 1 or 3-wire setups. Here's a pretty cheap one: http://www.amazon.com/DB-Electrical-AND0525-Alternator-Chevrolet/dp/B00KGIDF9Y
Less weight means less power drag to spin it, and 35-amps is more then enough to power your lights.