It looks like you are referring to the push rods between the rocker-arm tappets and the lifters (most likely, behind the cover plate on the distributor side of the engine, if you are talking about a 216/235/261)? One would not just disappear; however, I guess someone could have removed one. Typically, a push rod bends or breaks, or drops out of place due to a stuck valve. A head with stuck valves might be rebuildable (or even loosened up) but the best way to check this out is to remove the head and inspect it (or have a machine shop inspect it)..
You would have to inspect a variety of things in order to evaluate the usefulness of an engine.
Do you know anything about the history of the engine?
Was is stored outside?
What year and size of engine is it? You mention valve covers; is it a V8 or V6?
Tim
