For the aftermarket a few of the manufacturers give the alloy like http://www.delkron-mfg.com
For the stock HD it's mostly a general web search. There are a few good hits from welding and general casting sites. So far all have said 319. While that doesn't make it for sure, (why I said consensus) I don't see any info to the contrary.

A lot depends on what you are trying to do. Are you repairing cases that have cracked from stress, repairing corrosion or road rash damage, or modifying good cases? 4043 will work for some things and not for others. To do it right the case would need to be stress relieved and heat treated after welding, if this is a stress repair. On the other hand if this just making it oil tight again, because of some minor damage, then yield strength may not be that important. If you are modifying to correct a design flaw, then you might need greater strength than what it originally had. This is where you might use a different alloy to weld on additions.