Russ is right about the magnet glue failure on those models. I've seen a bunch of them do that. I've actually managed to seperate the cone from the surround, cut out the spider, remove the voice coil assembly, shim and re-glue the magnet, and reassemble the cone assembly with a new spider and put the whole mess back together. Probably more trouble than its worth, though, unless you just wanna make it a spare-time project...
As for models- usually, in those single-woofer models like a Monitor 5 or Monitor 7 variant, the woofers were 4 ohm. I believe the MW6500 and MW6502 were both 4 ohms, while the MW6501 and MW6503 (used in the dual-woofer models, ie, Monitor 10, SDA models, etc) were 8 ohms... IIRC. Any Polk woofer of that era and appearance, as long as it's 4 ohms, should work properly in there, AFAIK from working on them frequently in the past.
Regards,
Gordon.