"I" would be looking for rust on the cylinder walls--perhaps by dropping a videoscope into the plug-hole. And test the cooling system with a pressure-tester for cracks in the head, or an intake gasket leak that could have wet the valve.
No rust on the cylinder wall? No coolant leaks? Wipe off the obvious sludge from the valve, fire the engine, when it's at operating temp, shut it down and perform a leakdown test.
Either the valve seals and you're fine...or the valve leaks and the head comes off. If there's rust in the cylinder, the head comes off. Maybe the engine comes out.
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