Hmmm, that is a tough one. You might try a left hand drill bit and easy out to wiggle it loose.
Those factory installed pipe plugs can be a real bear! Installed extra tight, then couple that with swelling from rust and coolant corrosion, sometimes damn near impossible to get out. Especially on the vehicle. If you can get that broken ratchet out and you still have a good square left in the plug, I'd try a twisted square easy out like below. Probably have to heat the cylinder head around the plug quite a bit. I have removed dozens of those plugs for customers that way. Though mostly on the work bench not in the vehicle.
Best of luck!
