Was it overheating before you got it? Is the head/block discolored/burned paint? You won’t see any corrosion coming out of the water tube when probed that way, it is the actual smushed rubber of the water tube that may be blocking water flow.
OK, so the water pump parts are Sierra, you may want to invest in an OEM water pump kit just to rule out that possibility. We will see what happens when you replace the thermostat. Remove the overboard indicator plastic pan restrictor fitting just in case there is crud in there blocking the hose. Again, be sure the water tube isn’t bent and is fitting correctly into the impeller housing grommet.
Post some pictures of the powerhead so we can see if overheating was an issue before you got this engine.
