Copper is different than steel. It's not the heat that hardens it, the deformation hardens it. Heating and cooling anneals it, you can let it dry slowly or quench it either way it will be annealed if you heat it up then cool it off.
I have a vintage sled that uses a copper head gasket, I use permatex aviation number 3 sealant on it, per Graham Bell's book. I suspect the stuff is outdated and there is probably something else out there that is better for sealing, but it is readily available, and it has never given me any trouble. It doesn't harden, it stays sticky, so it is relatively easy to get the head off and clean the old goo off of it, much easier cleanup than say yamabond as an example. So I still use it. My motor is air cooled and gets really hot, not really a concern with a watercraft motor, but anyway that is my experience for what it's worth.
