I'm not sure about changing the seat diaphram, but I have added the foam to existing seat cushions just you described. It does help where the seat cushions have compressed with age. I have used two different types. I used 2 inch foam from a local hobby shop, medium density; it worked great. I have also glued old carpet padding together, three layers; using Krylon Spray adhesive. I have been happy with both methods. With both methods I used the adhesive to glue the existing seat bottom and used new burlap where the diaphram and seat cushion meet. It's not super strong bond, but it helps to hold it in place during reassembely.
My el cheapo method of repairing torn diaphram straps is to use tire intertube material on the strap ends. Mine seem to fail on the loop around the metal retainer. I cut off the bad strap end and form a new loop with the intertube sewing it to strap. Works for me and is a lot cheaper than new diaphram straps.
Sound like you are having a lot of fun with your GT. Good Luck!