This is quite the rebuild or should I say OCD restoration? I think you are to close to the project to be objective at this point - maybe some of us are as well - but we are pulling for you and the compressor!
Strange how seeing an old piece of equipment being restored makes you think of the good old days. When floor jacks had style and as a rule, compressors chugged along effortlessly, thup, thup, thup...
Fast forward to today and mentioning Edwards Truck Services & Body Shop in your thread puts your story on the fourth page of a Google search for their company...
They have an A+ rating with the BBB and no bad reviews on Yelp - the internet can be a powerful tool.
As you point out you have nothing to lose by giving them the opportunity to fix it - hopefully the owner's son will take pride in his work and care about their reputation.
BTW, I think you should name this compressor - that's right a name, that way when people say "after all it is just a compressor" - you can put them in their place and say no; this is the infamous "Legacy Saylor-Beall Compressor" of Garage Journal fame...or maybe just a simple name like "Jonathan's Compressor" - look what the name "Charlotte's Web" did for a little spider web in a barn. Who knows, with the right name there might be a book deal in your future.
Far fetched? Think about it; Charlotte saves a pig from butcher vs. Jonathan saves compressor from fiery, molten death at the hands of a scrapper...
You should have contest right here on GJ to name your compressor - after-all it would be fitting for the main character of such a saga to have a name...right?