It would be possible to build up the missing slinger and re-machine it, but it would be much easier to get another crank if you want the original setup. The crank may have had a Burtz seal, available from the catalog vendors.
I think that Tom Wessenberg had the rear slinger replaced. I wouldn't have thought that it could be done. I'm not sure if the spelling or name is correct but I think the guys name was Rich Faluka, please forgive the spelling if wrong .
Completely off or just down to the level of the thrust? Rich Fallucia at Antique Engine Rebuilding can restore it and I have been privy to his method and believe I could also.
Actually his name is...Richard Fallucca DBA Antique Engine Rebuilding. Duffy, you can google Richs phone # and talk with him regarding your request. However, as stated your crank may have been modified to use Terry Burtz seal...to prevent oil leaks. I use Burtz seals and they do not leak. Good luck.