http://www.partsforscooters.com/ATV-Engines-Complete
[ my question is for the people who has done the trikes what is the best size of motor to use on a build like this, and does any 1 know any sites for the 200cc plus sizes? ]
No I have not built a three wheeler.
I have used these folks for parts, but did not get a complete engine from them.
I just thought it may help you.
Myself, I'd look for a older branded scooter, whatever make your comfortable with, buy it, and use that.
I'm finding older Helix's, Reflex's for cheap money, maybe some I'd need to drive for, but a good deal is always worth a few miles to me.
Good luck in your build.