The serial number on the data plate is what they call the VIN now days.
A modern day US VIN is 17 digits and yours is not, of course.
The serial number is what the title should have on it.
My Willys CJ2A is a 5 digit serial number.
My 1930 model A Ford car is a 4 digit serial number.
If you use something else it will never reflect the serial number which is the production number too. A modern day VIN has country of origin, transmisison number, engine model number, finished trim number, and other stuff included with the actual serial number (porduction number) incorporated at the very end. Your vehicle does not have all this "stuff" incorporated as you are well aware.
Using the engine number is incorrect because engines are replaced and while not historically as valuable as a car or truck that have all the numbers matching, would really screw up your title as it would no longer have the correct serail number engine which would mean you would not be able to prove ownership because you no longer have the right car, which is "B.S."
Tell them that the manufacturer tracks the serial number on the data plate and that is what is supposed to be on the title.
Is this a vehicle that came from another state by chance? If it is then the other state's papwerwork should have the serial number listed on it somewhere along with the manufacturer, model year, and model number or model name.
Keep us posted.
jimm1009
PS: My CJ2A came from my uncle who purchased it brand new in 1946 and I am the 2nd owner. It still has the original engine installed although it has been rebuilt once and the engine serial number is now where close to the vehicle serial number but yet that is the way it came from the factory.