Last resort- - - -make a diaphragm. A piece of fabric-reinforced rubber gasket material the appropriate thickness can be cut to the shape of the old diaphragm and attached to the mechanical linkage in the center. If it's riveted instead of bolted together, any good machinist can drill and tap the shaft to accept a center bolt and some flat washers. Just be sure to Loctite the bolt that holds the assembly together. I used to make diaphragms for pneumatic valves that way frequently when I was a maintenance man for E&J Gallo winery.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!