Model 68 John Deere is older mower in which JD makes very few parts for anymore. If it was mine I would repair the governor if it was possible. Could the governor be out of adjustment or hooked up wrong? That model 68 mower had a over heating problem seeing they in closed the engine under a cover and the newer rear engine JD's have a open engine. If you do decide to replace the engine just have to make sure it will fit under the engine cover seeing that holds the seat up. That model JD dose a real find job of cutting grass but the biggest fault I find with it is that it doesn't turn very sharp. My advice is before you spend very much money on it would to buy a used mower that is a bit newer.