Hi Phil, and welcome to the forums!
While I work on the newer A/C units, I have never done anything to an Armstrong. As I understand it, Their set up is very similar to a regular home unit with process valves to add refrigerant if necessary. You never do this to newer units.
Anyway, Be prepared for spending a few bucks because this is what you are in for: evacuating all of the refrigerant (Freon) from the unit. removing the compressor and replacing with a new (rebuilt) one if you can find one. This must be properly soldered in place. Purging the unit with nitrogen to clean out any remaining contaminants, then vacuuming out the nitrogen, then charging the system with new Freon.
This is a VERY time consuming process and will probably involve the better part of a day. I wish I could tell you where to look for a repair. You might try a regular A/C shop as a start. I totally understand your desire to keep it original and perhaps a search on the Forum will provide more useful information.