Living in SoCal gives you a lot of choices regarding tracks and engine packages to race:
CalSpeed in Fontana
Adam’s Kart Track in Riverside
Apex Racing in Perris
I would recommend the KPV/HPV which is a 100cc air cooled engine. It is a high quality, very reliable, high revving two stroke (14,500-16,000+ depending on the pipe). Though a little slower than a 125cc TAG, the maintenance time and costs are a lot less. This allows you to focus more time on the two most important aspects of kart racing: driving and kart setup. I personally race this class so, admittedly, I’m slightly biased.