I think you need to be more specific, for example what year motor? late model KX's use a thin head gasket, however they make an optional gasket that is thicker, it was used on the 89 as stock and the thin was an option. The thick gasket is 1.0 mm and the part number is 11004-1186, on later models the standard is .4 mm with an option of a .2 or 1.0. Now if your working on an 88 or older then it becomes more difficult, I would have cometic gasket make me one out of copper, anneal it seal it and torque and away you go.
Updat the thick head gasket has been superceded, the new part number is:11004-1276