BigRock, I feel like it would be useful to add some comments to this thread. It could help others deal with a similar problem.
I am sure that the welder was stored for a while in a place where temperatures and humidity was not ideal.
I suspect that the reason why the Generator in your Welder didn’t have any output when you first tried it, had to do with the control board. Most likely, the pins on the board or the contacts in the slot had a buildup of oxidation and some of the pins were not making contact. That could be the reason why the generator started to work after you reinserted the control board. Then the pins made a better connection with the slot and that ‘fixed’ it.
Over the years, I have restored many pieces of electronics test gear, audio amplifiers and other electronic equipment. Some of those have been setting in garages, attics or barns for years.
Whenever I get a vintage, valuable piece of electronic equipment, before I apply power to it, I remove the cover and make sure that there is nothing burned or damaged inside. Then I clean it with compressed air. After that I unplug all the wire connectors and clean the contacts with isopropyl alcohol or contact cleaner. I do the same with any circuit board cards, contact switches, rheostats and volume controls.
It would be a good idea, as soon as you get a break in the weather, (April?) to go over all the contacts and connections and clean them good.
That welder/generator is a nice piece of equipment. A lucky find for you.
Ed