Craigslist is full of minibikes and go-karts, rolling frames or complete. Cheaper than building one, or a good head start. The chinese are pumping them out like crazy, so caveat emptor as far as welds etc.

When I was 17 I wanted a kart because we lived in a fairly rural area, and there was an abandoned rent-a-kart track I could use. For $75 I got a pro-made frame with slicks and disc brake, with a MAC-7 engine, a 2-stroke ported engine that put out almost nothing until 5,000 RPM. From there on it was (literally) a chainsaw engine, man what a ride!!

Check YouTube for shifter kart movies, I really really want to do that someday! Make a street legal one with a Sportster engine and full body...