An engine can give the impression that it's happy, initially in the flight, even though it may be running too lean, and if the fuel does not have enough oil. After a while, it heats up and then dies. Repeating this process will cook it. Combine this with a possible loose head, and the process magnifies. It may have been over propped for the type of flying you were doing, and that can also cause excessive heat. While it might seem obvious that the leaking tank could be the cause, this probably wouldn't cause overheating, the run would probably be erratic because of reduced pressure, but the engine shouldn't overheat. Lots of possibilities. Glen