I have been able to revive many over the years flushing with carb cleaner / brake clean and air. If the openings were not blocked prior to blasting /painting etc. there's a good chance they are full of debris. Pouring or spraying the chemical in, block the opening and roll the horn to get the liquid into the center. Repeat until you can hear the liquid inside the horn when you shake it.
Reverse and see how much cr-p comes out.
Repeat above if needed.
Counting the number of turns, Remove the adjusting screw, write the number down.. If it is dry, it's sealed. Squirt the cleaner trough the hole and shake. Repeat as needed.

Flush both sides with air and set aside in a warm area to evaporate any remaining solvents.
When no residual vapors are detected, Reinstall the screw the same number of turns.

Bench test. Sometimes adjusting the screw under power will wake them up.
The above method has worked many times but not always. Probably ` 75%|
If this does not work refer to the article suggested.
You decide which is easier

Not to hijack the thread but has anyone ever found a source for socket used to turn the adjusting screw???