I have been digging to see if we can get just the vacuum diaphragm but unfortunately I'm not having much luck. I do see there is a rebuilt unit with a core charge available here:
https://www.pelicanparts.com/More_Info/930602021FXOEM.htm?pn=930-602-021-FX-OEM&bt=Y&fs=0&SVSVSI=796
It seems on some aftermarket distributors they actually delete the vacuum advance/retrard
There's a thread with some decent info on it