Looks like foreign object damage to me and at a guess, I'd say it was caused by something much larger than anything that would have come throught the fuel system.

When you say the glowplug was "gone" do you mean it had come out of the head or had the filament disintgrated? What sort of plug was it? did it have an idle bar?
Is it possible that something has entered the cylinder from the bottom end? such as parts of a disintgrated bearing?

As Spetz said - if there is any damage to the liner -replace it too

The sharp edges on the damage to the head would need to be polished out otherwise you will probably get hot spots on them and possible detonation. If it was mine, I would replace the head. Are the glowplug threads stripped or is it just the way the photo shows?

Consider the price of all the spares you will need and compare that against a replacement engine such as the Magnum 52XLS