I just went out under mine with a flashlight and the calipers.

It's a 3/4" oil galley plug. OR, a 3/4" freeze plug would also work.

The good news:
It should be an over the counter part at any auto parts store.

The bad news:
You'll need to clean the hole with a rag, then tap the plug in with a hammer and a drift.

Yeah... that exhaust pipe is in the way...

...good luck!