The toughest 3 wheeler I ever rode pending is a 1985 Honda 350x ATC. Bored to a 393, stroked to a 430, midrange cam, 13.5:1 pop up piston. The bike was so strong, it would not stay on the ground at 1/4ths throttle. A major mean machine.
I am on three and only 3. Yeah 3wheeler.org is a great place to talk about trikes+toys. My ride is down right now but should be ready by next spring. Three wheelers!!! yaaaa.....