The camshaft-lifter contacts are the hardest working surfaces of an engine. Those will be ruined. It takes very little wear to get through the surface hardening of a camshaft or lifter and into the "soft" metal underneath. It's still pretty hard inside those parts, but not hard enough to last more than twenty minutes or so.

The camshaft journals and bearings are also under a lot of stress when pushing those valve springs. None of that stuff is going to be at all usable.