I've had very good luck at the CA DMV. I registered a Model 88 I imported from some time I spent in New Zealand. The bike was a non-matching basket, and the previous owner had since passed. There was ZERO paperwork, short of a bill of lading from import that didn't show the serial number of frame or engine. There's a special office that only deals with title issues and those folks were very nice and generous with their time helping me navigate getting a title in my name. They even made fun of the people at the Customs Office, saying that they were a bunch of psychopaths, and the title office decided an incomplete bill of lading was good enough, and I got the title in my name with no problems in the end.
All that was needed was an inspection by the CHP, with only a passing interest in the engine number, who was really just making sure it was vaguely motorcycle shaped, and the frame serial number was what I said it was.
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@swooshdave, I've found a good attitude goes a long way. Whatever attitude you walk up to the counter with is the one you get in return. Except at the Customs Office. They are a bunch of psychopaths.