They would be great if you could get them mounted, but what laws there are on motorized bikes always state that the bike has to be single speed auto transmission. No gears allowed, so you'd be running afoul of the law from the start. I think I'd rather use a Lifan on a mini-bike or go-kart or as a replacement engine on a tired old Honda CT110 trail bike.
I hadn't thought about the geared transmission thing - here in New Mexico there isn't a limiting law accept for the engine being under 50cc otherwise it is open from there. Of course you wouldn't want to push the limits of the law in ones area, but big engines and light weight vehicles (bike frames) is just plain cool. ha ha