Your car has two blower motors, one for the underseat heater and one for the defroster.
The underseat heater motor part number was originally a 5047615, later superceded by a DELCO universal replacement number 5047730. Heater case should be marked UH-20-52 or UH-21-52. The universal part no was used for many GM applications on 1939-54 vehicles.
A Chevy site www.cheviesofthe40s.com shows the same universal part for many of their 6V applications. They seem to indicate a 12V motor for 55-56 Chev trucks 5047849 can be installed in the older 6V heaters to upgrade them to 12V. They offer a reproduction motor made by VDO for this purpose. The 5047849 was later superceded by another DELCO universal part number 5044402.
For 1953, Buick went to a 12V system on the Super and Roadmaster while retaining 6V on the Special. The 53 12V underseat motor was a 5047699 which was supeceded by the 5044402 universal part also.
More can be learned by Googling the part numbers to find other possible suppliers who have these motors or replacemets for sale.
The heater switch has a resistor on the back of it to drop the voltage going to the motor on LO speed. Using the 1952 6V switch no 1998029 with a 12V motor will give a higher motor RPM on LO speed than the original 6V design. The 1953 12V switch number 1998018 should have the correct resistor to slow the motor down correctly.
The 1952 defroster motor is a 5047666 which was supeceded by DELCO universal replacement 5047733. The 53 defroster 12V motor is a 5047670. Was not able to find a universal part number for that one or any other cars that use that number. Vendors may still be able to provide something here.
The defroster only runs one speed, so no resistor is used.
The 52 6V defroster/heater fuse was rated 14A. The 53 12V fuse was a 9A.
If motor dimensions match between 6V and 12V numbers, the substitutions should work. Good luck.