[MattR]

Three ideas come to mind since it is getting hot enough to "weld" the spark plug electrode.

One idea is that the bike is running very lean (hot). Perhaps the airbox boot or reed valve boot are cracked. I doubt this is the case because if it was this lean than the piston would seize too.

Second idea would be the spark itself. Perhaps the ignition coil (part #2 in attached pic) is bad causing higher current (amps) to pass to the spark plug.

Third idea is a wrong spark plug was installed and the plug is running too hot.

These are just guesses, but it will be interesting to see what others here think.