Engine Conversions
It's not that hard to do, but you do need some knowledge of engines. I had a buddy who used to do it all the time. He would pick up a broken chain saw, blower of trimmer and in a few days have an engine he would sell. It takes a lot of hack sawing and grinding and you do have to make or buy a prop adapter. The mount is usually just an aluminum plate screwed to the rear of the engine.
I've seem him do 70 cc chain saws down to 15 cc string trimmers. There is a 50 cc or so, blower motor that makes a really nice RC engine.
Check on the engine conversion forum. There'll be plenty of data there. Some engines are easier to convert than others and you'll be able to find this out, too.