I believe it was Shurflo that was the producer of the Polar-Aire vents back in the day. I'm pretty sure they stopped producing them years ago so there's really no source for new replacement parts. So unless you can figure a way to repair what you have or find a replacement part at a salvage yard or parts source, I'd probably just replace the entire unit if it was me. I'm assuming it's a standard 14" x 14" opening?
eta: I just ran across this site in the Portland Oregon area that lists some parts for Polar-Aire. You might give them a call and talk to them about it and if they can't get the parts they have listed, maybe they can refer you to a place that can. Full parts list for all models is listed to in the service manual ...I'll attach it below too:
SHURflo Polar-Aire III - Catalog ...PDX RV LLC
(503) 305-6216
(503) 333-9585
.