Unless you remove the head, the drive plug comes up about a quarter-inch, more-or-less.
The existing O-ring is junk. It'll be hard as a rock, brittle, and not sealing any more. But you can't get it out, and you can't put a new one in there. All you have left to do is to cut and wrap a gasket coated in sealer under the flange, Clamp it back down, and hope for the best.
AFTER you get the head removed, the drive plug comes all the way out, and at that point you can break the old O-ring, stretch a new one back in, and theoretically that's all you need. The new O-ring is (supposedly) improved. When it was me, I put a new, uncut gasket in there for a "second" seal.
https://www.amazon.com/Permatex-80007-Hardening-Gasket-Sealant/dp/B002YLVLY2/ref=sr_1_2?dchild=1&keywords=Permatex+%231&qid=1591581753&sr=8-2