You could install a variable speed rheostat switch. https://www.ebay.com/itm/152383115222The modern day replacement motors from the repro parts suppliers are all single-speed. There will be two wires on these. Connect the red wire to either the high or low terminal on your Magic-Aire control. The black wire connects to ground.
I don't like the replacement motors. The one I got on my '55 Courier (also converted to 12-volt) rotates slower than the original motor did on low speed, and it takes forever to defrost the windshield.