Just bought a 1999 Terry 725R, when the previous owner was demonstrating its features he said "It has a bathroom fan" as he clicked the button, the fan blade shattered. "I mean, it HAD a bathroom fan....
Others can give advise here, but a lot of folks convert the old style fans to super quiet computer fans, like 100mm size. They're 12v too so you just need some modification skills to mount it and simply wire it in. The advantage is the lack of noise. They can be had for a few bucks online.
Other option is to junk the setup you have and go to a Fantastic Fan, or Maxx Air setup, both are great but that requires replacing the entire vent assembly. Great way to go though!