That is kind of tough in USA outside of California-as I never had a title to an MX bike until 1989 in Texas. Even then it was torture explaining to the tax office that the title was only legal proof of ownership, that I did not want an off-road sticker, or road registration.
That was a 1989 RM250, the only bike I ever titled. So, as you are looking at bikes-California is the best, not only since it almost certainly has a title, but also it is way closer to the shipping port(less cost). Most bikes in the 70s in Texas, Louisiana, Oklahoma that I know of-only had an MSO, Manufacturer's Statement of Origin-or in some cases a CO-Certificate of Origin. Typically when sold-the MSO was handed over with a Bill of Sale-signed by both parties. I think I recall having an MSO notarized at one point.
BTW, I undertstand that in California, you must have a title and green sticker or the authorites can confiscate( take) your bike, impound and destroy. That is, you can be pulled over and checked for this-then they can take your "gross polluter" and destroy it-giving plaesure to eco-nazis everywhere.
Well, maybe a few more in California may sell their bikes just for that?