Thanks for the tips.
The horns are really prety simple mechanically, they are a simple coil that magnetizes with current and moves a diapragm. The motion of the diaphragm moves a point type contact that opens and breaks the circuit. Repeat real fast and it moves air, and makes noinse. So if the coils and points are in OK condition, its not too hard to clean them up.
If they are seriously corroded or the coil is broken then no go, hard to repair. Have used small bolts in the past, but want a more "correct" look so want the rivets...
Thanks for the tips, contacted both.