How extensive is the mildew, and on what side of the board is it growing?
(1) I have microwaved softcover
RPG books for a
few seconds to reduce or eliminate the mildew smell. The books were in rough shape anyway, so I had nothing to lose. Maybe if nothing else sounds reasonable you could try the microwave.
(2) Could you make a color photocopy/laserprint of the mapboard and use it as a replacement?