You'd do better with a snowmobile motor (a la Tularis ...the watercraft motors are quite heavy, while the sled motors are much lighter given the same displacement. I for one would love a 700cc 2-stroke twin or triple in a streetbike, except for the fact that they present a huge vibration problem in that sort of application.
yeah, there was a carbon fiber framed 583 rotax/bombadier motored streetfighter looking thing in Twistgrip magazine. 310 pounds and hayabusa power level, with a CVT drive. The same guy made a feet forward version too. I like the way the jet ski motors look though, do they have transmissions? Looks kinda like a modernized GT750. I'd make a chopper out of it (I said lighter, not better).