Couple of fixes there. If we still have the part, it can be re-attached by drilling up thru the airhorn and tapping it for tiny screws. This fix is difficult, but effective.
An easier fix is to tap the thicker part for 6-32 thread, just a couple of turns, and install a 6-32" screw to hold the arm. The hole in the arm will need to be larger for the 6-32 screw. Tap the casting just deep enough so the screw gets tight just as it secures the arm flush, and retain it with loctite. The arm will pivot on the screw like it did on the roll pin.....Cliff