Note

Have an engine that is larger than initially required. Eventually its full power may be used for jump takeoffs and then cut back to a reduced level for flight.

Could lay the 2-cylinder engine on its side, put the shaft under it and use a crown and pinion rotor reduction.

Alternatively & perhaps better: Consider Hirth F 30 K; 4-cylinder opposed, 80 HP @ 5500 rpm; 103 Nm @ 5000; 1000 TBO @ 75% power; Weight = 92.7 lbs. Particularly if shaft and prop can go above engine.

And alternatively: Consider Hirth 3701 E; 3-cylinder inline, 80 HP @ 4500 rpm; 125 Nm @ 4500; Weight = 99 lbs. Particularly if shaft and prop can go above engine.