Idea...
That is a low-pressure area. Suppose you opened the plug, and let things dry. Put some JB-weld on the tip of a finger in a nitrile glove. Shove stuff into the crack from inside. If you are brave enough, you could suck on the outside of the crack to draw the JB-weld inside better.
I hope you can keep the threads clean. Then you can put the plug back in with teflon tape, and let the water out at the end of the summer next time.
An alternative idea is to apply a suction from one of the pipes with a vacuum cleaner. Apply JBW to the outside, and let it be drawn it. At some point, turn off the suction and goop some extra JBW.
If that is steel, you could find somebody to braze or weld a repair.
Of course if you can find a replacement, that will be more straight-forward.