Greetings Tom!
You have discovered one of the reasons that I had both of the original vent fans in my '64 Overlander replaced with Fantastic Vent Fans - - the cost of the replacement motors along with the fact that they move so little air for as much noise as they produce. The Fantastic vents are not cheap, but it is an entirely new assembly with a quiet, powerful fan. When the motors started causing problems in my Overlander the installed cost of a replacement motor was nearly $100 just a bit less than 1/2 the cost of the Fantastic Vent installed at that time. If you decide to consider a Fantastic Vent, you might want to browse the thread below:
Fantastic Vent Fan Thread
Good luck with your search!
Kevin