[sanddrag]

If you are going to take jumps, you should make the car as light as possible, but don't sacrifice durability. For the role cage, do you have access to a welder? Even just a small arc/stick welder should work. If not, maybe an oxy-acetylene torch for brazing? If not, then perhaps a simple roll bar (as opposed to a full cage) would suffice. Find some tubing that has a hole the right size to tap for a machine screw/bolt. Then bend a hoop (leave the ends straight) and tap both ends. If it is aluminum, it is possible that you could just cold forge the threads using a Grade 8 socket head cap screw.

What is the car going to be based on other than the pocketbike engine?
Basically, what off the shelf parts are you using and what are you making yourself? I'm insterested in exactly what the frame, suspension, driveline, and steering are going to be and roughly what the overall dimensions are going to be.