You could use a regular airplane four stroke, if you pay attention to some details. First will be cooling. Second will be gearing. Gearing will be important to prevent damage from over-revving the engine. Like the guys say you can damage an engine by floating the valves. Most OS engines are non interference engines and will not bend valves. By gearing it appropriately the revs will be kept in check. The biggest problem would be making a clutch work with the supplied shaft.

I have both the OS 26 and 40 four strokes in car versions. The 26 is little changed except for stronger valve springs, a different cam, short stubby crank, straight intake manifold, and a modified crankcase breathing arrangement. The springs, crank, and cam are available through Tower, so theoretically if you couldn't find an old stock FS-26S-C or FS-30S-C you could make one from the airplane engine. The FS-40S-C has all the above changes plus larger valves than the airplane .40.

This is a thread I started a while back. It contains a link I found on the FS-26S-C.
http://www.rcuniverse.com/forum/m_5482581/anchor/tm.htm

Saito has also produced a car version of their FA-30. I've never seen it, nor a catalog listing for it.