I'm pretty sure that it goes under "special construction vehicle". You would have to get a VIN created by the CHP. They will somehow affix the VIN to your chassis once it passes inspection. I believe it just has to have the standard lights, horn, possibly wipers and some attempt at a windshield. I think the CVC has all the regs. They smog it for the engine. My buddy had a 32 Ford with a 302 from some Ford in the 70's. It had to have smog provisions for the year engine.
Use old,pre-1968, VW bug tags, and use few parts that would suggest that that's what it is, like a fiberglass bug roof panel,windscreen, baja style hood...