Check out this cool mini bike Casey built from 1965 Popular Mechanics plans!

The engine is a two cylinder RC boat engine off of ebay, transmission is a centrifugal clutch with a jack shaft, tires and wheels are harbor freight, and the gas tank is under the seat, you fill it with a syringe.

If you go to his website, you'll get lost, it's so deep with cool stuff he's built, created, made, upgraded, etc...  racing carts, mini bikes, engine powered skates and pogo sticks, 55 Chev, 67 Porsche... it's loaded!  http://caseymoir.com