The old furnace is 48 years old. Suburban furnaces of that era were subject to recalls and those recall parts are no longer available. Few parts of any type are available. The safety issues involved with an old furnace are such that it is best if you simply get a new one. Take the "ugly gas wall unit" out and pitch it also. Then you can get at the space for the original furnace. Sizes have changed some on Suburban furnaces but the new ones are smaller than the old, so you probably can find space to put a newer unit in. Unless you are planning on travel in very cold weather, I would not put one larger than 24,000 btuh in, especially in your mild climate.
There are many advocates of repairing/restoring the original AC unit, probably an Armstrong. I am not one of those advocates. Yes, it can be done but it is going to be hard to find someone willing to work on it in any event. Searching for parts, and working on a 48 year old AC unit takes a special type of AC mechanic or person up to the challenge. The end result will probably be a more expensive unit than replacement and it still will be 48 years old. You could call around to see if anyone in your area would take the job on, but I am betting that they all will say no thanks.
And I am sure that others here will say it is easy peasy to repair an old Armstrong and advocate doing it. Ultimately it will be your choice as to what to do with it.
The furnace is a life safety issue, the AC is a comfort, cost, and hassle issue but not a danger.