That got really hot in just a couple of minutes, plus that plug looks not just hot, but lean too. I don’t think the mixture made it overheat, but it didn’t help.
The discolored area you mentioned is most likely caused by heat from the central exhaust valves, already a hot spot, with runaway temps it can really cook some parts.
I’d check the thermostat for correct opening, if it doesn’t have an air bleed port I’d drill a 1/16” hole as mentioned. It won’t affect the warm up but will let the air bleed out before you even start it up.
Check your water pump impeller, they’ve been known to slip on the shaft when revved up.
