Just about every used cub cadet needs a head gasket. The first thing i do when i purchase a cub cadet is change the head gasket, flush the carb and gas tank, clean and gap the points and ckeck the spark. If you look closely at the head where it meets the block near the exhaust valve in the front area you will see a black color (carbon) and some maybe wet with oil too. This tells us right off the head gasket is leaking. By this time its not a retorque since pieces of the head gasket can be missing. At this time you can decarbonize the combustion chamber and the valves too.
I believe that Kohler recommends decarbonizing the engine once a year(I think).