Better asked in the Engines/Electrical Forum, one forum down...
Since you mention Heat Exchanger, is this a Fresh Water Cooled (FWC) or Raw Water (from the water you're running in) Cooled (RWC) system?
If it's FWC, do you know if it's a "full" system (water/antifreeze cooling engine AND exhaust manifolds) or a "half" system (water/antifreeze cooling engine heads and block only)?
Has any raw water impeller fallen apart, sending rubber pieces through the cooling system?
Post all this back, somebody probably can help you.
It is true that a thermostat that's been overheated may not be able to fully open afterwards. A replacement thermostat can be automotive IF 1. It's Stainless Steel like Stant SuperStat and 2. Its physical layout matches the marine one in every way.