If your images simply had a magenta cast, then simply adding in some green would balance it out and you could be on your way. Unfortunately, you are not so lucky.
Your images are magenta in color because the other dye layers in the film have broken down. You have magenta information but have lost the cyan and yellow. Simply adding green will not suffice.
Given the average luminance value of each frame L1 and L2, you can calculate the ratio of the exposure values EV1 and EV2 as
EV1/EV2 = log2(L1)/log2(L2)
And see Wikipedia for more details on this formulation.