I own all kinds of these kits. I have built a few for people, that use them for commuting.
This has been the best out there, in my experience. I am 220. if you weigh more there is a bigger engine there
http://www.staton-inc.com/Details.asp?ProductID=3316
200 miles per gallon cant be beat!
of course the only drawback is that is doesnt work when its raining. for that I would go with a chain drive.