I get dumb ideas a lot, let's just put that out there now.
I am well aware that this ranks up there in the world of dumb ideas, but hear me out.
I can get my hands on a Lifan produced replica of the Suzuki AX100 Engine
for about 480 bucks delivered to my door.
I think this would be a fun engine to put in a gas bike frame.
It's not silly powerful. 8.25hp, fairly low compression, and a 4 speed gearbox.
Outside of potential chain alignment issues it could be made to work. The only real draw backs I see are needing fairly wide crank pedals, if they would work at all, and the weight of the entire motor assembly itself at over 40lbs.
As I said, it's a dumb idea, but oh so tempting.