Looks like every once in a while, game collecting karma deals me a decent hand.
I got my "untested" Fami Disk system (I think it was a steal at $32 WITH an AC adapter) last month to find that it had a melted belt.
Got my replacement belt today, and followed the www.jfgoods.net guide to replace the belt and re-set the magnetic disk-reader head.
First I replaced the belt and tested it out and repeatedly got Error 22 which I believe is "can not read disk sector 01", so I went back in and did the head-adjustment, and when all was said and done, I've now got a cosmetically great condition Fami Disk Drive that boots games in like 3-4 seconds flat!
Total cost on the system, about 12 disks, and a replacement belt was well under $100. I think that's pretty good based on what I've seen on eBay or elsewhere.
While it wasn't the hardest repairs I've ever done, it certainly wasn't the easiest - and if anybody ever comes across a unit that powers up and works electrically/electronically (I'm a klutz with a soldering iron), I'd be more than willing to help out with belt replacement / head tweaking if said person in need of help is willing to get a replacement belt and ship it all out to me.