Hi
Go to chainsaw engine repair shop , find a Stihl 025, 43 or 44 cc engine, engine is atached with only four bolts to the plastic case, but you need to make a carbureter adapter, smaller Stihl are same system I dont know if bigger Stihl are same system. I have one 025 and it is very easy to put on a engine motor mount .I have not converted it yet , need machining of the rear part of crankshaft because I will put it to move the prop.It have more diameter.And after it I put a CDI ignition .
To make the things like it is a bit expensive and not simple I dont know how many time and money like you spend.
Go to the site
www.rcfaq.com
Go to the engine conversion see the photos, good explanation
Have nice time
Jose