I bought an exact replacement - just for the electric motor and fan blade part - on eBay for both of mine - well under $200.
I did the work in an hour for both from the attic.
If you can get inside there and get model numbers for the old one and maybe post a picture, I can try to find one for you on the internets.
http://shop.ebay.com/i.html?_from=R40&_trksid=p5197.m570.l1313&_nkw=attic+fan+motor&_sacat=See-All-Categories
(more expensive than when I got that pair several years ago, you might find cheaper if you dig)
At that point, you can DIY or hire a handyman do put it in for you.
I'm getting rid of mine when I replace the roof this summer. Passive venting, done right, is much better. The fans I have now suck the AC out of my house through the walls because the intake vents have been painted too many times. This house was built long before energy efficiency was such a big deal. Plus, those old fans were letting a lot of stink bugs into my attic until I covered their openings with hardware cloth.
Edited 1 time(s). Last edit at 06/04/2011 12:43PM by wnrsm.