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Though they would have done that as metal bits in the water system aren't good.
@Light_EDDed get a wet and dry vac to draw all the metal bits out, flush the system with a hose pipe, feed the water from the rad so the water exits from the area the the shards went in. That should clear the area out.
You might have to cut up the water pump, remove the impeller and head. Then use the body of the water pump bolted in place and the broken bolt hole as a guide to drill the broken bolt out. Use motor oil to keep the drill bit cool and sharp.
Do you have access to a welder?
Can you heat the area? Hopefully heat will help move the bolt remains as it is not seized in.
Be gentle with easy out as breaking the easy out is bad.
Can you use the drill to make the hole more conical, that way you can hammer a slightly larger torx bit in to the hole? Try to see if that would help get the bolt out. If that gives no joy then, is there enough space for an impact driver and hammer to get it to move.