What you intend to do is illegal, because you would be registering the car with false documents

The way forward if you don't know the original registration number would be to send a letter to Fiat UK with the Vin number, ask them when the car was manufactured and ask if they know the registration number that was issued to the car.
Then the car needs MOT-ing on the Original Vin/Reg number, Once you have MOT - Insurance you can register it again at the DVLA either on the original number or an age related plate