I guess I would be looking up the law in CA.
The VIN is what's used in most cases. That is usually a sticker on the frame.
To see the engine number you have to remove something to be able to see it.
Do they require engine numbers on 4 wheel vehicles?
Is the Engine # and VIN # on the registration for the vehicle?
Any chance you could contact the seller and ask if he owned the bike since new?
If not, does he have previous owners info? Might be worth asking.
Have you owned other motorcycles in CA? If so, was this always the case, that they needed engine #'s?
If first motorcycle to register in CA, ask around if that is standard procedure to require those numbers.
Back to my first statement - look up the law - Sometimes the cops do things just because they can get away with it, if someone doesn't know the law.
Educate yourself - if you don't know for absolute that is the way it's gotta be...
JMHO
Maybe Pashnit will chime in. He lives in CA and should know what's what...