Two keys are missing from my Mono/Poly. Can anyone please help??? They're two white keys, normal C and A. (Alternatively D and F keys would do the job too.) Thanks for any help.

Korg sells replacement keys directly. I've never had this problem, so I'm not absolutely sure that those particular keys are still made, but its reasonable to assume that the
keys themselves have always been of a standard size. PROBABLY!
Check out korg.com and go to the accessories section. That'll tell the tale.
Good luck!