I just went through that exact same situation on a trailer that had never been registered and had no paperwork other than a hand written bill of sale, no title. After many trips to the CA DMV they finally told me to get a signed letter from the seller stating that the trailer had not been registered in any state for more than 5 years. The CA DMV then issued a new title using the existing VIN number.
On another trailer many years ago I had the same situation and I welded and ground the old numbers off, painted the trailer and registered it as new home made.
either way works.