That’s a shame. 😕
The only thing that I know with certainty is that seeking help from Winchester is a waste of time. Sadly, Winchester today is little more than a brand name. The company, machinery, and workmen that produced your father’s shotgun exist only in memory.
With no real certainty I believe it’s safe to say that almost anything, including your shotgun, can be repaired if you find the right ‘smith and you have deep pockets.
I am NOT a handy, fix-it type guy, but I always stand back in amazement seeing what can be done in the hands of a true craftsman. Especially welders. Those who somehow manage to make a whole out of bits and pieces.
Back in the ‘60s when carbines and Garands were scarce, cut and smashed receivers were made whole. More recently the same has been done with the receivers of a wide variety of guns imported as cut up parts kits. It’s AMAZING what can be done.
Given the relatively low pressures involved I bet that your dad’s receiver could be welded. Finding the right welder and being willing to pay his fee, that’s the hard part.
Good luck. I hope it works out.