I went thru this with a 37 Dodge PU.....no vin tag and truck titled by the engine number and the engine was long gone.

As I found out after buying a shop manual, the frame is also stamped with the vin. It took about 1/2 a day at DMV to get that all straightened out but I now a legit paper on it.

Now if a sharp LEO decides to check numbers on the truck, which do you think he's going to go by, the stamped numbers on the frame or a tag that's held on with 2 screws? You can do what you want, but personally I kind of like knowing before I put a bunch of money in a project that I actually own it with good paper.

Laws vary greatly from state to state but finding a frame number and talking to DMV is where I would start first.