http://www.weberprecision.com/
How about something like (perhaps slightly larger) powering a bar stool or a yard kart?
Here's a magazine that specializes in this stuff:
http://www.modelenginebuilder.com/
I've been to a couple of the shows they put on and it is very, very cool looking at, hearing and smelling what these engines do. Walking in the building at one of these shows, you get a sense that there is a lot of knowledge and fun to be had. Then someone fires up one of the model V8's and all rational thought stops for a moment and you turn into a kid again.
Enjoy!

