Pulling the valve cover won't help you, lol! Some of the valves are probably broken off, and there is probably some damage to the pistons. Like I said, people just find a used M20 and swap it in, since they're so cheap. Decent engine with decent miles for $500 all day long.
As far as other stuff to worry about, not really. I'd just look for rust and solid, up-to-date mechanicals. It's a given that many of the hoses and lines will be old...same for bushings and rusty bolts. Old car, use common sense. Generally a Hennarot car that doesn't look faded has been well taken care of, which is why I say that's probably a good buy.