I think there are a few people here interested in adding small gas engines to bicycles and I thought of you when I saw this - Ettore Bugatti designed a 'cyclemotor' power unit during WWII and, as you would expect, it's a flight of fancy - 12cc, double overhead cams, and a supercharger! With a piston of less than 1" bore, the 15,000-18,000rpm working speed doesn't seem that high......
Bugatti T72 article
Bugatti T72 gallery
Andrew