How to replace flue plate on cast iron stove with sheared bolt?

The rear flue plate on an Elle Wood burning stove cracked and fell off. I have a replacement however one of the old retaining bolts has sheared off and is now stuck in the hole.

I need to either drill out the hole or drill a new one next to it. I then need to tap a thread into the stove body as there is no access to hold a nut still.

What kind of drill bit do I need to a) drill out the old bolt? or b) drill anew hole in cast iron?

How do you tap a thread in cast iron?

Thanks,