I adjusted the the valve lash yesterday and noticed that one of the rocker cover bolts has sheered off at the head. Someone has drilled the center of the bolt and stuck something in it which has also snapped. So now I have an unknown metal object stuck inside a bolt which is stuck in the head

So pretty much, I need to redrill the hole and tap it. What's the thread sizing for the rocker cover bolts?
Cheers,
Jimmy.