You might try spraying the warped balsa with ammonia (from a trigger type spray bottle) and clamping it in place until it drys. This may have to be repeated a few times, but might work on an already sheeted wing.

I have a BVM F-16 kit that I bought from a guy who had it sitting in the shop for awhile. The trailing edge of the wings and ailerons were warped. I used the same technique, and they are now straight again.

Good luck,

Jeff