If it's one of the Tantalum-type "blasting caps" then there isn't
much of an issue, provided that the board isn't damaged. It's
probably a bypass cap, so the drive might even run without it, and
there is quite a range of things that will work just fine. Pull one
that's close (and meets the voltage ratings) off of one of your scrap
boards (don't worry, Tony - nothing too nice in mine, mostly PC stuff).
Logically, since I don't approve of board-swapping, I should approve of
raiding boards for components... Needless to say I'd not strip a part of
a rare classic computer to get a capcaitor, but as you said, generic PC
bits are fair game :-)
However, I would be very supprised if that capacitor was not available
new from the likes of RS components or Farnell. In that case I'd fit a
brand new one, rather than one that might repeat the pyrotechnics...
-tony