It looks like the crack is on othe outside of the water jacket and hence would only be subjected to a maximum of about 14psi of pressure. Personally, I'd get it tig welded and then grind the weld smooth that that it looks normal. Often you can find welders who will do small jobs like this fir $15-20.
If you can't find a good welder I don't see any harm in filling it with jb weld. If I am looking at the crack correctly, the worst that will hapen is that it will slowly leak antifreeze.
good luck