If you had the original motorcycles vin # and plates you could just keep buying tags for it and put them on the build. You would just need a bit of frame with vin#.
The biggest hurdle would be an inspection on the street, for sure you will get stopped. I guess if you had registration and tags and all the safety equipment worked you should be fine.
James and Colleen Tucker.
Aut viam inveniam aut faciam
Beat our time, Los Angeles to Ushuaia 70 days