You don't get a new plate unless you can show all the major components are new (you are allowed to use one used part which must be reconditioned to as new.)

If you use a new shell and sufficient parts from a donor car you get an age-related plate appropriate to the donor.

If you use a used shell, you either use that car's original reg and just get on with putting it back on the road or, if it doesn't have a reg you get a Q.