Though it may not be the world's most practical motorcycle, you've got to admit that a highly-modified XR750 just sounds like it would be a ball of fun.
This particular XR has been punched out to 900cc's. It uses XR1000 rods, two front heads, and a modified oiling system along with an S&S crank pin, pinion shaft and sprocket shaft, along with Mikuni 38mm flat-slide carbs, Hagon shocks and lots, lots more.
It's not street legal (of course) and it's probably way to loud for anything but the track, but if you've got the dough, we're sure you could find a way to have fun with it. (Who doesn't need an excuse to learn the fine art of flat-track racing?)