might as well use a snowmobile engine if your going to make the swap, because you will need the belt setup to run it. by the time you ad the clutch setup, wiring, battery/charging system (mainly to run a electric water pump) you have a big heavy clunky looking bike. with today's technology and Calvin's cylinders the power can easily be reached with a "banshee based engine" idk right off hand what the larger engines 18+mm DM engines are making, but you can come damn close to 120 on a full built 421 cub motor. go on up in size and i am sure money will buy you way more than your ready to use at your average dune fun drags.
a lot of ppl have went the engine swap route and have realized it was a waste. hell i think there is even a video of a turbo busa banshee getting raped by a larger stroked banshee based engine.