There was never a gasket for the valve cover, they were machined to sit against the head with nothing between it. I'm more then sure i read that here on one of these posts, think J6 was the one who informed them. Might be in the book to but best solution is either resurfacing the parts or use Honda bond or yahma bond ....
I assume the heat eventually warps the metal and it looses shape over time/ heat and pressure.