Doing more research fer the spring suspension...here's what I've come up with so far:
Original solid rubber puck, 2" thick, 3.75" diameter, 5/16"x1.5" studs
(Current) Goodyear Air Sleeve Spring, 1S3-013, 1.5" height (compressed), 3.6" height (expanded), 3.2" base diameter, 2.6" top diameter, 5/16" top bolt, 3/8" bottom stud/air port, 100psi (max)
Here's what I've found so far looking at airless airbag/springs:
Firestone Work-Rite, no suitable products that can be fitted because of the hollow core design
Timbren Axle-less suspension, potential in the 400lb rubber suspension part, 1" deflection when compressed, less than 3.5" wide, approximately 4.5" long, approximately 2.5" tall, unknown core
SumoSprings Solo, solid core, 5/16" stud attached, potential to cut down and attach a threaded insert to secure to the bottom, manufactured to various heights, densities, and diameters
The SumoSprings Solo has the biggest potential to replace the Goodyear airbag spring provided I can get some specs on the models they have available.
SuperSprings Self-Adjusting Suspension Stabilizers > SumoSprings-Solo