You could also try for an 'age related' registration if you can prove you've used at least two major components from an old car, such as your Pinto engine and gearbox -
https://www.gov.uk/vehicle-registrat...erted-vehicles
This is the route that allows some of the very expensive Jaguar C and D Type replicas to run with a period registration.
You don't need to know the reg from the car that gave up it's engine, you'll just get a new, unique registration from the year the engine was built.
Good luck.