My text didn't come through, so I'll do it again.
After searching the internet, making numerous unsuccessful phone calls and reading all the advice I decided I had no other option but to try and do it myself. I spent twenty bucks and got a sheet of 1/8 neoprene off Amazon. Then I made a wooden ring and clamped the rubber onto the back of the pod. I centered a small coffee can and a five-pound shotput in the middle and slowly (took about ten minutes) heated the neoprene, allowing the weight to gradually deform the diaphragm, I didn't really think this was going to work but when everything cooled the neoprene held most of its shape. The membrane was a little stiffer than the original, but I thought I'd give it a try. I bolted the whole thing together and using my vacuum pump applied vacuum to the booster. Much to my surprise it retracted 3 inches and worked perfectly and is so powerful I could not hold the shaft back with both hands! I'm going to do a little more experimenting with 1/16 neoprene and 1/16 silicone but now that I've got the plan together, I think I've solved the problem. If I was a rich man, I suppose I could have spent a few hundred dollars shipping these off to some high-end rebuilder/restorer but the fun of successfully doing this myself was priceless.