If that doesn't work, is the part easily removable from the rest of the torso? There might be a bunch of Ozformers with junker Megatrons that have that part intact.
The problem will be that the part was probably removed on Hasbro's Megatron's. The firing gimmick its self was removed by Hasbro so I'm guessing repair will be easier than waiting for Takara G1 Megatron junker to show up.