We've given a lot of attention to the Smartuki here but there were some early motorcycle powered cars with interesting designs, too. The BMW Isetta and Messerschmitt KR were around in the 1950's and early 1960's. Long out of production, Tritech, a company in the U.K., started making replacement parts so owners could keep them on the road and restore their former glory. After a while, they were making so many parts they were able to reproduce the entire cars, along with some donor parts from a Mini. The new models are called the Zetta and Schmitt. They use the engine from a Honda CN250 Scooter, and in the case of the Zetta, you can also use a Kawasaki 500.
I remember these cars being around long ago and while in high school and working as a mechanic after school, someone came in with an Isetta that needed work. The door opening the whole front of the car was cool. Don't remember what the problem was but we had no parts source anyway. The car was definitely very neat.
What's really interesting here is this ties in directly with what I was saying about motorcycle kits and components over the past two days. A company takes an old, out of production vehicle and begins making replacement parts and finally can recreate the whole vehicle. Old motorcycles would be perfect candidates for this sort of thing, I would think, and a company able to do this could eventually branch out into modifications to adapt newer engines and running gear. Perhaps they could begin building better frames for the old style engines. It depends on demand and skills, but the idea is there.
Tritech link via Retro Thing
see also: Motorcycle Powered Cars