for all practical purposes, any small engine from a hand held tool will work,.......it's just a matter of how much junk needs to be cut off the engine block to get rid of the "not a aircraft motor" looks and what needs to be done to put a prop on it. some engines don't have a very "prop adaptor friendly" crankshaft sticking out to get an adaptor on and most prop adaptors have to be custom made now-a-days ..simply because the popularity of converting has passed.
that said, if you have your own lathe or access to a lathe and can use one,....the adaptors are easily made with basic lathe processes. the adaptors are really the "make or break" issue, in most cases... having a legitimate "machine shop" make one for you, would just about over price any conversion compared to buying a conventional aircraft motor of similar size these days..