From the severity of the damage in the photos, i'm suspecting that it was either a loose washer in the magneto housing or it was the woodruff key that keeps the magnet in alignment with crankshaft indexing slot.
I have had this situation happen to me before, when the woodruff key slips out behind the magnet as it's being installed it on the crankshaft. If you are aware that this can happen, you are paying attention that the woodruff key can slip out.
If the woodruff key did indeed slip out, there's a change it could become trapped in between the spinning magnet and the magneto arms. Considering that there is only 0.5mm air gap between magneto arms and magnet, the problem will become quite serious when a 4mm woodruff key tries to pass through a 0.5mm gap.
the replacement is going to be a slant plug if I go that way. Will it be better quality?
Higher compression = higher engine torque, with some people also reporting that these engines produce increased power due to the slant cylinder head.