As a welder by training, I agree. You're not going to get that little nugget back on in such a way that the threads work. Buy a Nickel welding rod (Ni99 or similar) knock the flux off with a hammer, and use an oxy-fuel torch to build the area back up. I recommend the oxy fuel torch because the less concentrated heat will be less likely to crack the block somewhere else. Once you're done, cover the area in a rockwool or fiberglass blanket to keep it from cooling too fast.
