My brother has a late model, (with the "smog?" slots in the cylinder) Ryobi 31cc on his Ryan's Rebel. Stock carb that has been "massaged" slightly larger, muffler gutted and provided with a larger outlet. Besides that, no other mods other than the conversion to aircraft use. It turns an APC 18 X 6 wide blade prop at 7500 rpms, and flies the 80" span, 13.5 lb. Ryan's Rebel quite well. It won't do "3D", but it WILL loop from level flight at half throttle.

So... Take it from there. Nobody wants to recommend something and be blamed later for their advice if it doesn't perform to your expectations.

Good luck and have fun,
AV8TOR