Your only other choice for a real serial number would be to check out the surviving chassis/body/equipment list to determine about what year the Jeep was manufactured, according to various production changes over time, then see if you can obtain an approximately correct serial number from a scrapped A1 and use it for the data plate and DMV listing. If you have a legitimate serial number new plates can be made, but it's not kosher to just make one up. What is the current serial number, according to the title or any other surviving paperwork?
The correct serial number would start with MD, followed by (at least) 5 digits, the earliest of which would be somewhere around MD10000 to MD12000 (June? 1952) or so, and the later ones having a serial number some 90,000 to 100,000 later- like MD100000- going into the later 60's or maybe beyond a bit, I don't know- very, very roughly as I don't have exact start/end build dates or serial numbers. Remember also there's no correlation between similar-sounding engine and body numbers.