The clicking sound may be from the valve lifters sticking, or not pumping up. If it’s still there after using the SeaFoam equivalent, it may be camshaft lobe(s) is/are worn.
You can remove the valve cover, and measure (and compare) how much each intake valve stem rises, and how much each exhaust valve stem rises as the engine is rotated by hand. The figures will be the valve lift. Compare to each other, and to the specs in the repair manual. If the wear is excessive, the camshaft and valve lifters will have to be replaced.