Originally Posted by
Jhieminga
Heading off on a tangent a bit, but the ill-fated Bugatti 100P replica used motorcycle engines. The accident report highlighted that it wasn't the engine but the clutch/transmission/gearbox that contributed to the power failure and subsequent crash. If your engine runs up to 10,000 rpm, you're going to need some way to get those revs down to below 2700 for your prop.
The Rotax series of 2 cyl in line 2 -strokes generally had a max of 6,500 rpm and were fitted with 2:1 or 3:1 gearboxes; my Monnet Moni had a KFM 107E flat twin which also had a max of 6,500 rpm but was direct drive ie the prop span at 6,500 rpm too!