[Greg Reid]

Sorry, I didn't know that was directed at me.
They are actually pretty simple and usually the problem is crusty or rusty diaphragms.
As long as the coil has continuity, which you would check with an ohmmeter between the prong and the body you should be able to get it working like new. If you don't have a way to check that it's okay because it's very unlikely that that will be broken.
The way it works is this- There is a coil of wire surrounding a metal, movable core so it works like a relay.
When you press the button current flows through the coil moving the slug. The slug is attached to a thin metal diaphragm so that it gets an oil can action when the coil magnetizes and moves the slug. There are a set of points, one point contact is fixed and the other is attached to the movable diaphragm. When current flows it pops the oil can one way and as soon as it moves it breaks the contact so that it relaxes the diaphragm again but as soon as that happens current flows again and it pops back. This happens at a high enough frequency that it makes a buzz. The buzz is amplified by the horn body. That's it.
All you have to do is grind the rivets out and make sure everything inside is clean with no dirt, or rust so the diaphragm is flexible. Clean everything up including the contact points, replace or fabricate a gasket to seal the two halves together water/air tight and use screws in place of the rivets.
After doing that you can use a current meter, which is what is pictured in my post, rigged up to adjust the current for peak value while the horn is being energized. There is a screw with a jam nut on the horn that you do the adjustment with. If you don't have a current meter you could probably do it by ear. Just adjust it for the loudest clearest tone.
Mine are still working and looking great.