i would have to say it is burnt oil and fuel. If it got hot enough to discolor the head it would of broke down sooner then that. could be leaking a little around the muffler seeping through the gaskets a little. spewing some out of the carb. do you fly a pattern or burn holes in the sky. I know my engines have a lot more gunk on them that are in the planes that fly a lot of maneuvers. especially stall maneuvers.