Jaames,
Since the subject of flanges has been covered, I might stick only to my area where I actually have a working knowledge. Disaster recovery.
A bumbling home handy man like myself, and evidentally you too, should have a roll of silicone tape. Also known as emergency or rescue tape, it is always on hand at my place to "finesse" those little issues that arise due to poor planning etc.
You can get a roll at Bunnies for $15 I think. You can check it out here:
https://www.rescuetape.com/silicone-tape-uses
It is rated up to 200 celcius and may well work to fix your poblem. Wrap it around the threaded nipple and it will weld itself into shape and cover the hole. Put it right against the kettle wall.
A simple, but possibly effective method to fix a leak as described, is to go buy a roll of plumbers' tape. Get the yellow stuff that is rated to, I think 200 degrees. Roll a long strip, fold it onto itself once or twice and and then roll it up and down on your leg to make it into a sausage shape. You repeat the process until you have built up a fairly significant amount, then coil it around the threaded nipple until it covers the hole. It is an old plumbers trick (I worked for Reece for several years).
Good luck and....... as you say, measure once, cut twice!