I've faced the same problem and have managed to bring back 5 motors to life of perhaps 8, by replacing the brushes. The "lost" 3 suffered either from terminal commutator damage or cracked magnets.
I'd open the motor and see if the brushes are the probable cause, cheap enough from Simon's friends if you are in the UK and don't have to pay the extortionate overseas postage.
I've also replaced a few fans for local owners, buying the new fans once available on eBay. The ones with the spindle press fitting rather than the bayonet type, plus resistor. But I see these have all gone, shame, the price wasn't too bad about £200.
I must admit I'm not a great fan (ha ha) of these universal fitting kind. I like something that fits properly, perhaps Mat at DMGRS might see an opportunity here
Good luck with those very rusty torx screws, ruined God knows how many bits in the past.
Regards