That looks like a serious let-go Iain.
Jetted for 12,000 revs?
Might a failing piston/rings/rod be the ultimate source of the misfire? Must be about the only things you have not changed.
I would assume the misfire could cause the failure but is the other way also possible without an almost instant failure? It seems odd (to me) that you have been able to run with the misfire for so long and no other signs of impending problems - like dodgy oil pressure.