Re: Can I use a motorcycle engine in my boat?
Hayabusa engines are used fairly often in small cars / large go-karts and seem to work fairly well.
Of course, this still isn't the constant high load that a boat will demand, so I'd be very diligent about cooling and monitoring the temp.
I'd definitely vote for liquid cooling, it may be easiest to just reuse the bike radiator and fan, but it would be a cleaner install to replace the radiator with a water/water intercooler and pump, so you can use raw water to cool the bike system.
A 'busa wouldn't have shaft drive though... maybe a u-joint in the prop shaft to level it out a bit, then a double width chain to couple to the existing countershaft sprocket? This would leave the engine sitting sideways in the boat, which isn't exactly the best looking solution, but it would be fairly close to centered, which a shaft drive wouldn't be.
Never thought of putting a bike engine in a boat, but I've had plenty of daydreams about a sportbike engine in a small roadster, something like the Ariel Atom.
It's not too hard to find wrecked sportbikes with low miles either, engine is usually fine, with thousands of dollars damage to the plastics and frame.
Another engine that may actually fit your needs better is an old Goldwing, GL1000 or so. Horizontally opposed four with shaft drive, so it would center up pretty well in the boat. A lot less power (and more weight) than the Hayabusa though, and I was presuming that performance is a fairly high priority.