For starters, all the Jap bikes ran like a fine-tuned watch. The Buell, although very, very powerful with the low-end torque with it's 1200 motor, was very clunky, and rattled my teeth out. Because of that, I don't think I will ever buy another Buell again.
I have ridden a dirt bike with a Rotax engine, but didn't like the configuration of the kick start, because it was on the left side. The dirt bike did run out very nicely, though. Now if this Buell/Rotax engine fixes the rattle your balls issue, then I would be happy to give it a look.
Anyway, my 2 cents....