Yeah, about that...
I have no idea where the rest of the electrode went.
After I finished the bearings and my new exhaust (unrelated, but don't make a full pie-cut exhaust out of stainless... it suuuuuucked), I chucked some oil in it, cranked it by hand with the plugs out to check and see if there were any tight spots. Then put it all together and started it.
It idled fine, firing on all 4, etc, etc.
The next day, I pulled the plugs out again to make sure there weren't any tight spots.
There were none so I put them back in.
It must've broken after that since that was when it stopped firing...
I checked inside the cylinder for the electrode, chucked a magnet in there, made a cone out of paper and stuck a vacuum in there, and found nothing...
I guess the electrode isn't in the combustion chamber, at the very least...
Also, the porcelain is fine and still completely intact, so I'm stumped as to how it happened...