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@TheShipwright
I've linked to two diff types, the second link is traditional type of screw extractor and you'd need to be able to drill into the centre(ish) of the bolt and secure the spindle on the other side of the BB to use it but this is standard approach to damaged bolt/screw heads.
The first link is a Torx extractor, it "might" work if it can get enough bite and it's big enough to fill the gap.
Otherwise drill the bolt head and heat the BB spindle and you might be able to extract leftover bolt stub. If you know somebody with a mig welder they could weld onto the bolt stub.
Sorry all engineering /metalwork type approaches from me.