I have been in on a couple block repairs. One was so bad about 12 sq inches had fallen off the side of the block due to it not being winterized before it froze.
We ground into the crack or beveled the edges of the fall out piece and wire welded it into place or filled the crack with weld. Then we cleaned around it very well and used JB weld to seal it tight. Those boat engines are probably still in service today.
We never tried to blend it in, but if you are going to paint that is a great idea.
