An easier way might be with a sharpened pin punch. Lay that flat against the outer bore and cut the plug ring. Once you have it cut, it should let go, being able to contract from the bore. Might take a little tapping around the perimeter to break the rust loose. If there's a little of the step cut away, that won't matter, but try to leave the outer bore un-scarred, as that's where the seal happens. Definitely use some non hardening sealant around the bore when you install a new plug.
