12 v squirrel cage fan motor

You could try Grainger for the motor.
https://www.grainger.com/category/motors/dc-motors

Have you removed the motor yet? I helped with an M9540 and the motor was molded into the plastic in such a way there was no way to replace just the motor. I know that sounds irrational and is challenging to most creative folks, but we ended up online and it cost $510.00.

This may be what your tractor needs. https://www.ebay.com/itm/Blower-Mot...560397?hash=item1efbebe7cd:g:O8QAAOSw529dirQ3

You can go to https://apps.kubotausa.com/illustrated-parts/ and agree to the small print, then enter your tractor model and search for the AC components and come up with a Kubota part number. Then start your search.

Take it to a place that rebuilds electric motors. That said it may cost more than a replacement, just bought a new 3 phase as it was cheaper than a rebuild. Try Johnstone if Grainger doesn't have it.

yikes 200 to 500 for a lousy motor !, perverted to Canucks that's 300+ !!!
I'd be bodging a $20 unit out of a scrapyard car..
I assume you've confirmed the motor IS dead(on teh bench) and not some silly open fusible link or bad switch ??