The aluminum Edson quadrant (it's round, not really a quadrant) can get fixed to the SS rudder shaft by (bimetallic) corrosion. In that case it's not coming off without a cold chisel or a Sawzall. And then it's flat-out replacement. I was able to save one on a Superstorm Sandy job; but it meant all new parts from Edson: cable assembly, connecting eyes, everything else. When you do get it off and go to reinstall it, use quality marine lithium grease or even Tef-Gel on it. Don't spare the grease.
If you have the shaft assembly out, be very circumspect about inspecting the condition of the stainless. Any sealed-up internal stainless pieces get horribly pitted-- even so small you can't see it-- and the next step, unlike with aluminum or even plain steel, will be snap! --sudden and catastrophic failure. I hate to spend your money; but your boat's nearly 40 years old and you're probably looking at an all-new rudder assembly including the shaft.
I would not waste time in boat junkyards as the rudder you think is serviceable will be as old as the one you have now with the same potential problems. If you're adamant about saving as much as you can of what you've got, consider grinding some of the bad stainless and welding in new bits. You'll want a pro for that. The fiberglass is easily mended-- you can get it to close enough of the original shape and then fill and fair as much as you have to for it to be better. It's not vitally mission critical; it just has to turn the boat. But use quality marine-grade materials. Typically the blade of the metal structure gets 'glassed to one side; then the other side is 'glassed to that and the edges well lapped with fiberglass tape. It's not that difficult if you have even a basic knowledge of fiberglass repair. If I were there I could do this in my sleep.
Don't bother with replacing the foam. It's worthless-- adds nothing but weight, holds water and mold, fools you into thinking there's some 'strength' there, but there isn't. If your stainless steel is rotting, no doubt it's due to the foam holding water against it. At Cherubini we evaluated this and determined that fitting a little bronze drain plug into the bottom (not the fitting, just screwing the plug into a hole) is a better decision than replacing the foam. So long as you've got ample skin thickness where the blade is 'glassed to the shell, it'll be good enough.
As for the Edson parts, call Edson directly and talk with Will Keene. Tell him I sent you.