Duck, your exactly right, ya have to have the rod that matches the case alloy. The rods are easily obtainable by any welding shop that competatent to do the job. Its not a big job either. Depending on the location of the crack it can even be welded wthout disassembeling the engine. Most of the WWII vintage cases are a magnesium alloy. Some new engines are the same aluminum you will find in automotive aluminum engines. There is nothing out of the ordinary about welding the cases, same practices as welding any cast metal. preheating, cooling and temperature control are all pretty much standard, as is surface preparation.
Hope this helps some. I would expect a lot of A&P folks to tell ya to get a new case when the one ya got can easily be repaired though. They are trained for aircraft safety not tactical repairs. So your kinda on your own there.
Scotty