To change tappet cover you have to remove injection pump, intake, and stock fuel filter if you have it. Short cuts will have it start leaking again Usually the gasket gets brittle and drys up, leak goes down side of engine in rear or front, hard to see where it comes from. ECM bolts to front of cover. A trick is to clean cover really well, use 3M Weather strip adhesive and glue it on edge. Gasket only goes on one way, read label. The factory service manual for my year 2002 does not even show the tappet cover. You can only see part of cover with all the stuff in the way.
Rick