I agree w/ Ethan, if it's an MCS/TPS setup. If it's not, then you have to worry about whether it's single MAB or dual MAB. Otherwise, the M4M airhorns are somewhat forgiving, esp once you hit 1977 or later.
How badly damaged is the air horn? If the material is deformed around the flaps you can probably file it away. If it's physically bent, that's another story...