Your problem is that even if you find a replacement slide to fit the original bbl it's problematic that it will a) fit correctly, or b) last.

Your existing slide was cut down and welded, which presumably is where it cracked. ANY of the welded slides are suspect, and cannot be considered reliable shooters.

The catch is that your bbl has the large muzzle (to fit the bushing bore of original, cut down, slides). I'm trying to remember if there were any one-piece (proprietary castings) slides made with the large muzzle cut that would be even remotely good, but I can't at the moment recall.

If you find a large-muzzle non-welded slide you'll have to make sure that it's got good bbl lug engagement into the slide lugs, AND doesn't have excessive end play (checked w/ fully assembled gun, slide just slightly back from battery position).

Unfortunately I suspect your only real recourse is to a) find a complete later top end to use on the gun, or b) possibly find a replacement welded slide - and retire the gun as a memento of your father. Good luck (meant sincerely, not sarcastically).