For other forums, theres BARF (bay area riders forum) and south bay riders--a quick google will get you the urls.
For single track, the closest you're going to get is CA OHV parks--Carnegie and Hollister are the two closest, plenty of fun but a "park" nonetheless.
Is your 450X plated? You may have issues as a now-resident of California, but that's your decision
Good real single track can be found up in the Sierra--places like Georgetown, Foresthill, Gold Note Ridge are within the upper limits of a day trip, but I normally take the camping trailer and make a couple-three days out of it.
AMA District 36 has some resources for riding as well.