You & me both, but they are really expensive now. People over here (UK) have added Banshee snowmobile engines to some of these. They are based on what was the RD/RZ350 from the mid 80's, but can be taken to 500 or 550cc, with 80-90hp from a leightweight parallel twin 2-stroke !
My plan was to supermono the ZZR, but I liked the little Ninja emgine so much I decided to keep it. I have Versys-X 300 which uses the 300 Ninja engine, so the big bore plan was hatched to try to make 40hp at the rear wheel. We'll see how I get on