I've seen one with a VLX600 motor in it. frame's kinda spindly for that IMO.

to get em legal you either have to go thru the "custom motor vehicle" thing thru DMV (pain in the ass), or hack off the neck, and weld on one from an existing bike with a clean title (the legalities there are arguable, but "technically" is legal, DMV only cares that the VIN matches your paperwork, the bike will then be registered as whatever the neck came from, and insured as such)

here's the link to thier site. Basically a bike-in-a-box. don't think you're gonna get on the interstate with it though, you MIGHT break 55mph on the bigger version.

http://www.kikker5150.com/hardknock.html