Unfortunately the metal disk in my horn (I am calling it a diaphragm) looks like swisss cheese.  Does anybody have any leads for someone that might have such a part?

Yup, Rog is the man.

Also worth checking out antique car swap meets, quite often find same thing with a longer trumpet on for reasonable price, especially if partially dismantled. 

If Roger cannot help you, let me know, I will have a rummage in the shed.