I would say the engine sat around too long and the push rods and maybe the lifter oil holes have gotten glogged-up. I would take the rocker arms off and remove the push rods. Blow shop air through each on and then re-install them. Then spray some carb cleaner or brake cleaner down the push rods. Put the valve train back together setting the valve lash as you go. Start the engine and see if you loosened up the cloggs. Or take the intake off and pull each lifter from it's bore and clean the lifter and the push rods with shop air and carb cleaner.