If it was a leaky head gasket you will see that brown line on top of the gasket surface extend from the barrel to the outer edge, you will also most likely hear it leaking.
Did the piston have a lot of black carbon deposits on its surface? If not i would look at the carb, take it apart and clean it.
I do not believe it is your carbs, because you said the engine did not heat up more than normal. But just clean them, you have been riding without an tank cap.
Then when you put your engine back together take the copper head gasket to someone to heat up uniformly with a torch to soften up the copper (let is cool slowly in air).
Also make sure the engine is not sucking air.