I left karting in '92 at age 47. I was racing what Joe raced, but the last 2 years I went back to "sit up' karts on the road tracks. Slower speeds, very competitive and a little more reasonable in costs. Back in the 80's and 90's IKF (International Karting Federation) was the preferred sanctioning body west of the Mississippi. That may have changed.
http://www.ikfkarting.com/
I see the Nationals for sprints are close by in Santa Maria and the road race Nationals are in Portland at PIR.
Start off with some used equipment that is 1-2 years old. It takes a year to be competitive and you don't want to waste a year of karting progress with a new kart in your first year. There are schools at most tracks. I'd do the school in a rental first, if possible.
There is a LOT of tweaking to be fast, just like any other racing. As a driver, you have to learn to be as much dead weight as possible when driving sprints. The best drivers sit in the seat like a piece of stone with only thier hands moving just a little. It doesn't take much input to go fast. But, it takes a lot of endurance and strength, especially sprints.
One other form of karting is dirt ovals. I never did that, but it looked like fun, especially in the bigger bore and dual classes.