This has evidently been floating around for some time but I just came aross this link over the weekend. Looks like a fellow over in Germany has taken a racing kart and added a bit more power in the form of a 1988 GSX-R 1100 engine with 115hp, the GixxerKart. There's a video of the build process followed by some action at the track. 0-60 in under 3 seconds with a top speed of 105mph. There's also a more recent version powered by a 1994 Honda CBR-1100 XX engine of 164hp. The Honda powered version was built as a two seater. Pretty cool. There's more info here at the builder's site which also shows his handiwork on a number of Minis and other assorted motor vehicles.
see also: Motorcycle engine powered cars