I'm not an expert, but I've been doing autocross events for 15 years, so I'd like to think I know quite a bit. If we are talking about SCCA autocrossing, there are a few things you can do in stock class. You can run any air filter element, spark plugs/wires, and catback exhaust. For suspension, you can run any stock-type shocks with stock springs, change the front or rear sway bar (but not both), and run any DOT approved tire that fits on a stock size wheel. If you go above these changes, it will put you into a different class where the car may or may not be competitive. Let me know if you have any questions.