My guess is that tarnished will not affect it's operation. I have had a little corrosion on the electrical terminals that can make the aquastat act a bit flaky. Also, the 120v terminal somewhat "fragile" on mine and I had to modify the circuit to use the 240v terminal wired to the 120v input. If you can't troubleshoot it then I would almost tend towards another entire unit (depending on the cost of a new probe) - then you will have a spare at least if the old one is still working. Good luck.
should be on the bottom of the aquastat. I'd check flea-bay. they go for around 50 bucks. I use one aquastat on my boiler. 2 aquastats to me is just one extra thing to go wrong. if your handy with wiring, you can set it to run off one aquastat. (just throwing an idea out there)
if you are looking for an aquastat you can likely find one on the pdf link I posted. you can pick the options you need for your application
