Scooter engines would be a good choice. *I haven't seen any whole scooters that are too cheap though, and for the same price might be able to get a whole quad instead. *The other option is the Honda Silverwing. *That has a more standard CVT setup, but the crank is splined instead of taper or keyed so you need an adapter to run a different primary clutch. *A few years ago when I was in school the snowmobile team used the Silverwing engine in a sled with a turbo, made around 80hp and was pretty reliable. *I put a pic on here before...here it is:
