I built my car from a new shell and got a new registration number, age-related to the age of the donor, and new VIN.

Keeping the donor's reg and chassis numbers were NOT an option.

The first part of the VIN is used to identify the manufacturer and plant, reshells get a DVLO (or something like that) prefix.

Remember, the reg number is not really the car's ID because reg numbers can be swapped around.

The Vin is the bit that decides the identity of the vehicle and, despite stories that Ford dealers and bodyshops used to stamp the original VIN into new service shells after severe body damage made the reshell necessary, I'm not convinced it was ever legal to do so even though we know it was fairly common practice.