There are a couple of guys that restore horns. They are fairly simple except they are held together by rivets that need to be drilled out and then you must fabricate new ones. I have not found a source for those yet. Just a coil inside that has probably opened. If you dont care about originality, you can just disassemble, rewind and pop rivet them back together.
If you want a really LOUD, but cool sounding horn, find an old 6 volt "high" horn, and put it in place of your stock 12v one. Once had a 57 Pontiac that I put an old Ford 6v high horn on, and it was a beast!
Mine had deteriorated to a low moooooan. I replaced 'em with a pair of european PIAA hi-lo's. Obviously, they're not stock, but they're black (plastic, though), they're shaped pretty much like the stockers (with the stickers removed, at a glance they could pass as stock) and they sound just like a bloody Ferrari! (and a LOUD one at that!)
Not for everyone, I know, but an option...