Broken key screw on rear of M3

[Euler66]

I just acquired an M3. It's probably been out of use for 25+ years, so I've added oil and am patiently waiting for it to do its work. However, there is also an issue with a key on the upper manual. The hex screw in the rear that keeps it (the key) on has a broken head, so only the stem remains and it is flush with the organ, so I can't use needle-nosed pliers to back it out. If I were to somehow push the rest of the screw through, it would end up inside the organ. What would be the best way to fix this? I'm comfortable disassembling things as long as I don't have to unsolder/solder wires.

[KC9UDX]

If you can get hold of a left-handed drill small enough, you can drill it out. With care, the drill will bite into the screw and unscrew it toward you.