Scout around online and you can find all sorts of scaled down car engines that run, this one looks like it runs on compressed air, one of the neatest I've found was a R/C 'T' bucket that drives and is powered by a very small Ford Flathead engine. People who have WAAAAY too much time on their hands.

Wasn't there a company a few years ago that built and sold running (or close to running) 1/4 scale display aircraft engines? As I recall they weren't cheap but were pretty cool