Build a go-kart using readily available parts and a lawn mower engine. This is a very simple design built with junk yard scavenged parts mostly. Remember web uilt this as kids, the dimensions would alter somewhat for bigger riders.
The GK2G (Go Kart 2 Go) is a concept Go-kart by BEAU REID that can actually fold in half allowing the racer to chuck it safely in the trunk of a car. Reaching a high speed of about 60mph due to its low weight carbon fiber chasis and powered entirely by high output lithium batteries.