You should always replace the valves in a rebuild. At least the exhaust valve, but for the price I just do both. Unless it's a K161/181. The intake isn't available aftermarket, and Kohler ones are not cheap!!
Governor gear, I'm on the fence. Normally I don't unless it looks bad or I know the engine got real hot. Figure the old Kohler one is better than the new ones. Probably cheap plastic on the new ones.