As Jim said, if you can mount a prop on it then an engine is an engine. I don't think the huge heat sink head will cause any problem either because the heat won't have much chance to travel very far up it before it's cooled to a point it's not working anymore. For the .12 I'd try a 7x6 prop, an 8x4 for the .15 and maybe an 8x6 for the .18 (or thereabouts). You won't be able to use the car muffler though.

That said, my son broke in his .28 car engine by running it around a field for a few tanks starting at a 4 stroke and gradually leaning it out (flame suit on ).