In Washington I was able to registure a 1947 Doodlebug as a motorcycle. The lady at the vehicle registration desk at the local court house phoned Olympia [the state capitol] for guideance. She had a frown on her face. I went there prepared. Had the little mini inspected by the DMV/Highway Patrol and got a paper listing the serial numbers on the frame and the numbers on the engine. The nice lady, true to form for minor funtionaries, did everything possible to stymy the process. But to no avail. The great state of Washington had a listing for the post war Doodlebugs that said they were at one time licenced for the street as factory equiped. Took the better part of 20 minutes. However, as it is a vehicle that did not originally have turn signals it will go on the street with only head and tail lights and ONE rear view mirror. No stupid worthless turn signals and bozo mirrors. When the registration was complete the nice lady, as a parting shot, said "You will need a motorcycle certification on your drivers license to operate it". As it is over 125 cc I can even legally operate it on a highway. It's a motorcycle! But around town is all it will get.
So, my advice is to acquire a mini, it will be a vintage one, that was offered, back then, from the factory with lights that were legal, back then, and allowed that particular model to be regerested for operation on the street, back then. All you will need is a frame with numbers. Put a vintage engine and wheels and bars on it, if there's non there. You must show a [appearent] complete machine, not just a frame or collection of parts in the back of a pik-up truck. Get a signed, dated bill of sale the states the make, model, year, and serial numbers of frame AND engine. Go to DMV and with a straight face tell them you have a bill of sale but the guy that sold it to you long ago lost the original title. They [most states anyway] will inspect the machine and "run a make" on the numbers to see if it is stolen. If no red flag they will fill out a form that allows you to registure it. Here in Washington the state will registure it in your name but will not issue a "title" in your name until three years pass. That's not a problem. If no one claims it during that time it's yours to request "perminate" title. But you will be riding it on the street right after you get it registured in your name.
What is needed is for a learned mini bike historian to compile a list of the makes and models that were originally registureable way back then. Armed with that list a guy could fine a described machine and strut down to the Highway Patrol/DMV and show them your stuff. I'm telling you that all that is really necessary is the proper vintage frame with numbers. You can put on whatever era engine and wheel that suit you for the registration process. I used the engine that I fully intend on useing on the completed machine. how about it guys? Does anyone feel qualified to start a list of minis that were at one time licensenable?
I have not yet got my antique Doodlebug ready to ride. I have been looking for enough original parts to completely restore the little girl. But I now have grave doubts that three grand won't be enough loot to get the job done. They are availabe but none of it is cheap when you find it. So it's beginning to look like I'll end up with a "resto' machine that will fool all but the trained eye of a Doodlebug collector. I can hardly wait to show up at my favorite biker watering hole with my Doodlebug.