Unless you have a thing for strict originality, I would replace the entire vent/fan with one from Fantastic vents or Maxxi vents. Both are popular and have much better venting capacity at much lower power draw than the original fan. By much lower, I mean 1/3 rd to half.
If you must have the original, I will bet that there are many out there which have been replaced with other fan systems. Someone will volunteer one for you most likely.
Also, one of the parts places, like Inland RV or Out of Doors Mart may have the original motor, but it will be costly.