My buddy built a 3 wheeler like that, years ago, using a Honda liquid-cooled V-4, that he bought cheap with a wrecked front end. I don't remember the model, but it was 1000cc or more. He grafted the front end of a dune-buggy frame to the Honda. It was pretty quick, in fact, he had a severe traction problem with the motorcycle tire. He machined the spokes of the mag wheel, and modified a VW rim to fit, so he could run a flatter, wider tire. Sorry, no pics. I'll ask him if he has any that I can scan.
The 3 wheeler he built after that one was a 350 Chevy "sidewinder", one speed, chain drive affair, with a REALLY FAT rear tire. It hasn't been on the road in awhile, but he still has it. I can probably get pics, if anyone is interested.