Quit running 90 octane. It's running too hot. The reason it likes 90 is it's running lean. Richen the carb up. Put a new plug in it too. Hope you didn't hurt the engine yet. You have a pretty bad "hot spot".
Pull the gasket off the head, and lets see some pic of the head without a gasket on it. Better check the plane on the head.
How would you know if the piston is scored? You didn't pull it out. You can't see if it's scored with it in the block.
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