I too have a zbox engine.
The latest batch of engines from the new factory are ripper! They use needle roller bearings and very precise also the seals are improved on the older ones.
Be very careful when you take off the engine head that you dont 'twist' the cylinder at all or that the muffler under tension pulls at the cylinder. The gasket seal between the engine and the clylinder (bottom gasket) must be sealed perfectly, if this ever leaks the engine will run lean and dry. order new gaskets for when you clean inside.
Warren is very helpfull and has all spares