When I converted from 6V to 12V I did and extensive search to find a fan motor that would drop in with minimum/no changes. This is what I found. You can order a replacement 12v motor from AutoZone. Below is a link to the replacement motor. They cost about $32. If memory serves me the shaft might be slightly longer but it fit with no modifications needed.
Make sure you do not reverse the wires. This motor is reversible by switching the polarity when you hook it up.
Link to AutoZone Electric blower motorI also just noticed I have the only review posted for this motor on the AutoZone site.
I upgraded my 54 Chevy truck to 12v and needed a replacement motor for the heater. This was a perfect match to fit the original squirrel cage and bolt pattern. It also has a reversible motor that allows you t configure the rotation based on the design of your fan. It also support my two speed fan speed switch. I have been running the fan for 2 years and works great.
