replacing with a used engine is also worthless as you dont know if its already dead. Better choice is to set a base point where the manufacturer wanted.
e.g. - the sheer number of tappet setting jobs that are done at "lathe" shop in Pakistan is a sign of owners stupidity and mechanics not knowing what the are doing.
The correct way of setting these tappets is to replace the adjustment shims to correct thickness. Not skimming them down on a grinder.