On the 420/560 motor it is most likely that the problem is a rocker arm to cam issue. It can all be done without removing the head. Removing the rocker arms to view their surface would be the first step after removing the valve cover. The removal is facilitated by the special spring depressing tool.
There will be no way to see the bad part of the rocker without removal. If the cam is rough you won't need to remove them for a diagnosis but it will require a cam. MB says that cams and rockers should be done together. In the field the rockers are replaced without the cam, at least till the cam is rough.
Be sure the cam oil tubes are tightly mounted into the cam stands.
Bosch Master, ASE Master, L1
33 years MB technician