I've come across a Stihl FS80 brushcutter engine, 25cc.

Is there any hope for this engine? How much prop will it turn?

If it can be converted, I might just build an airplane for it.

I can't speak to the FS 80, but I have a FD 108 (35cc) on a 19# Bipe. The engine is running a CH ignitions syncrospark unit. I raised the compression and cut the ports for 105 deg ATDC for the exhaust and 108 deg ABDC for the intake. It turns a 20-8 wood MA Semitar at 6800. There was a fair amount of cutting to get rid of the excess metal on the engine, but it made a good conversion.