If the unit is metric, then the Allen head screw I’d bet it’s a 3, 4 or 5mm screw. Try your local hobby shop that sells radio control cars and airplanes. They sell lots of metric hardware. If you find one but it’s too long just cut it down using a Dremel with cutting wheel. A shallow head mike be hard to find.
PS, it’s rare but not impossible that it could be 3.5, 4.5 or 6.5mm but that’s usually used.