You can get a nice little thing called an "oxygen sensor simulator" that feeds a signal to the ECU to make it think there's still an o2 sensor hooked up.

As far as your choice of exhaust system and the fact that you're in California I would perhaps stay away from a test pipe. A high flow cat and a real catback exhaust with mandrel bent pipes will get you more gains than a header and test pipe bottled up by crush bent pipes and the stock muffler.