Make sure its not stolen first
I spoke with a gentleman last year that had just gotten his R 50/2 back after it was stolen in the '70s and some unsuspecting individual had bought it and tried to register it.
Yup the original owner was shocked that 40 years after his pride and joy was stolen it was found, it was in good shape and it was being returned to him by the cops.
You should have the VIN run before yo buy it. Even a title doesn't guarantee it wasn't stolen a number of years and owners prior to you looking at it. The gentleman I spoke to told me his bike had been passed between several owners and had a valid and clear title in the name of the guy who sold it last.
Do your home work first, there are several states that only require a notarized bill of sale on vehicles older than say about 20 years old. You get the bike registered in one of those states and then transfer the registration to your home state and request a title for your bike. Like everything else these days it's not easy or cheap but its a waste of time if the bike has a bad past.