Does it have good compression? If so, pull the back plate. If it's clean, put about a tablespoon of SAE40 motor oil in there and close it up. Pull the valve cover and check the clearance. Put a couple drops of same oil down the pushrod tubes and on the touching moving parts. If it's a mess inside let us know what you see, a pic would be good.

Use OS F plugs. There are others, but I wouldn't bother.