This is my bike. It was a regular mountain bike when I got it for $23 at a goodwill years ago for this conversion. It has a 50cc Chinese engine based on the Russian Type D motor.
Top speed is about 20mph, fuel economy is 100mpg.

It does generate a good bit of pollution for what it is, as it's a 2-stroke engine, but I don't really care. It cost me less than $200 to build.

Kinda a bit of a maintenance queen, you have to really give it a good checking over before you go out and ride it, breakdowns with these are a pain in the ass if you have to walk it somewhere with a flat or the chain breaks/clutch cable snaps. Still fun.

100% legal in Arizona. Requires no license to ride, is exempt from registration and emissions laws, and requires no insurance. Legally considered a bicycle and can be taken anywhere a non-motorized one can, except for the bus/light rail due to the gasoline tank.