Here in WA state, you can take a vehicle that is a blend of parts to State Patrol with the paperwork you have for both and they will give you a clean registration and mark in the federal files what was done so you are covered. I had to do that with an "abandoned vehicle" in order to use the drivetrain from my other car (which was rotted out) They gave me a 3 year (sometimes more, sometimes less) registartion only and after that time, if no-one has filed a stolen vehicle report, you are issued a title.