I think the bad thermostat diagnosis by phone should not be taken seriously. It's unwise to take blind stabs at resolving the problem without doing some testing first. And you are right -- a universal replacement will require some rigging with included brackets, etc. to make it fit, and I would be skeptical about such a part even working properly.
You're facing a common problem, and I had to let my nice '57 GE Combination go because I couldn't find anyone willing to replace the compressor, and it wouldn't have been cheap either. If you just need a new filter/dryer, that's something that wouldn't cost much to do -- if you could just find someone to do it.
The thread I linked above indicates there's someone in Indiana who made the repair. I don't know what part of Michigan you're in, but you may need to consider driving the fridge down and letting this same guy get it working properly again.
One thing you may try is if you're on your local NextDoor board, post an inquiry to see if anyone knows of a refrigeration/HVAC person who would be willing to diagnose and do the work. Maybe some retired guy still has the required equipment and can help you out.