it could be shot. If it has power when it's cold that would be because the sleeve has cooled and contracted to form a good seal around piston. providing adequate compression for power. When it gets warm the sleeve expands revealing the loss of pinch and compression. There is not a mark gallon wise that signals an engines death. Many things can happen that we are unaware of on a forum that could mark the engines death.
Try the suggestions above hopefully its a lean bog.
Good Luck