Well first things first you need a new metal plate abit bigger than the hole in the carpet with your chassis number stamped on the just cut the old one out and weld in the new one.

ill give you a warning now police will impound the car If thay pull you over and find the chassis number has gone off the shell as its what happens to stolen cars and then sold on with new plates and new vin numbers....

as for cost in a garage less than £100 supplied and fitted.