Hey Nick, have you thought about a snowmobile engine?
I saw a neat toy over at my neighbour's place yesterday. One of his kids bought a fix-me-up go-kart, powered by an air cooled, 2 cylinder, 2-cycle snowmobile engine. Probably between 350 and 550cc by the size of it.
Completely home-brew machine, which used the donor sled's CVT to link to the rear wheels. If you ask me, a CVT beats the heck out of a fixed gear ratio.
I've seen beat-up-but-running snowmobiles selling for next to nothing in the classifieds on many an occasion.
I doubt Ray would mind if I took a couple of photos and posted them, if you're interested.