Check with your local racetracks, I'm sure there might be 1-2 in your area, call them and ask what age group you'd fit into, go to a few of the races and make friends, you might be able to purchase an old kart and parts from them and start from there. You're a bit older than the smaller classes which are less expensive but you still might be able to be somewhat competitive for a few thousand dollars.
If you just want to have fun, check out dirt track stock racing where they pick a stock motor and nearly yard karts and modify them.