I remember seeing a photo of a .61 two stroke belt driving a large prop on someone's pattern plane, back in the days when F3A rules limited two strokes to .61 (10 cc) and four strokes to 1.2 (20 cc) displacement. It is my opinion that unless you are restricted to a certain displacement by competition rules, it's better to just direct drive the prop with a larger engine. (the KISS rule, keep it simple, stupid!)
I even prefer direct drive on my electric planes.