How bad did I screw up

Weld up with nickel rod, braze, or use a JB weld type product re drill and tap.
Welding and brazing is a PIA due to keeping such a large area preheated throughout the process as stated.
When drilling a mill would be nice so you could resurface the mounting face as well when done.
I would imagine a few cus words were emitted as a result of the breakage.
Experience in a given process may be the deciding factor in the repair. If using the original piece tig may be the answer?
Old engine cores are getting harder to find so successful fixes are important.
Maybe sonic check the block before doing the repair depending on the future plans of the block. Could have a thin area on a cylinder or two? May not be worth fixing?