A 20 oz engine?
If I understood that correctly, he meant it required an engine that weighed 20 oz to balance correctly. Check out the Saito 100, I think it's real close to 20 oz. Spins a prop big enough to clear the cowl, too.
A .52 (four stroke) would probably fly it at close to the speed of the .46 FX version.
Good luck,
Dave Olson