Try this site:

http://www.motoredbikes.com/index.php?

And I disagree. Although some appear to be appalled at the idea of putting a motor on a bicyle for high moral reasons (or something), there is a whole bunch of folks who are tinkerers, build-it-yourself fans, motorheads, or whatever who like to fool with such things.
There are a wide variety of engines available, both four-cycle and two-cycle. They run economically, better MPG than your average "scooter".
Also, many of these self-produced bikes show high style and are often made to emulate early motorcycles.
Such things are just fun to build and run.