The Trident seats may or may not work. That first stage had 3 or 4 seat changes in it's life, these changes required you also change springs and other parts as part of a kit. You MUST get the correct one to go with the springs and other parts you have as the Trident seats do not come as a kit with springs and other parts. The real issue you have it the seat. All of the orings are common types and the diaphragm in the fist stage is common with US Divers/Al regs so you can get those as well. I have restored a couple of the 400/800s and a 900 or 2 just for fun.
The second stages are a different matter. All the parts to repair them are common with other regs so new parts are not a problem. The are actually pretty good second stages.