I had a stihl 21 or 22cc that I converted. I never put it in anything though. It was heavy for its size,but made a nice conversion. It ran Super good and was one of the best engines I ever had to tune.

Actually....heres the thread I started awhile back about it....

http://www.rcuniverse.com/forum/m_82...tm.htm#8216368

Its probably only going to be a good engine for something with very light wing loading like a cub or something like that. My engine was an FS-81 too.