You can get polepiece screws from WD or Allparts - not sure if they're imperial or metric, but it should be fairly easy to find out.
I'm not sure it's a simple swap, though - as Agent Orange says, slugs are pressed in (and may be difficult to remove) whereas screws go through the bobbin, then a spacer and right through the baseplate. I'm not sure if it's the spacer that's threaded or the baseplate.
On the Gibson 498T/490R pickups the baseplates do have two rows of holes for screws to pass through, but if I remember right one set of holes is 53mm spaced and the other is 50mm spaced, so the same plate can be used for either string spacing.
(Bottom line: I'd settle for just removing the covers and leaving the slugs as they are.)