Drill exhaust manifold bolt - Remove head or not?

If you don't have an oil consumption problem, I would leave the valves alone, seals, maybe. Just my opinion, valves and rings should go together, overhaul.
Depending on how the stud removal goes, use a very good sealer on the new stud, let it set after installing, cure time before antifreeze.
Definitely remove the head and find a machine shop.