I'm not so sure this would have been caused by a Bendix or drive pinion failure. The whole purpose of that housing is to contain such failures.
I think it more likely that the engine
was dropped during installation
. Perhaps even onto that very large and upright bolt in the foreground of your first pic? Looks like maybe an engine
An errant blow with a hammer might also do it. Perhaps during starter replacement? Or even some over enthusiastic prying with a pry bar? Maybe by someone trying to pry the engine into place or alignment?
As noted by others above, stop drilling the cracks would certainly be a good idea. If you can get a good angle on them with your drill.
And I'd be keeping an eye out for a replacement casting. In the mean time, I might want to fabricate some sort of cover just to protect it from the elements.
It should be fixed. Because it is the diabolical nature of such things to wait for years, maybe decades, for just the right moment, and then...