RE: Drop Plane... Decathalon 120
Any 50cc gasser that I know of would be too big for that plane. You would need so much lead in the tail to balance the plane it would lose all its good flying characteristics.
A large glow in the 150 or 180 class would work, or a 30cc gasser could work. Try to choose an engine that would weigh no more than the 180 glow.