I wouldn't have an engineering problem with cutting another key way as I dont think there'd even be close to a strength issue there but that'd be unrelated to the current crack situation (as said toss it & definitely Kudos for catching that as most wouldn't as it is such a trouble free/stout piece & unless I am missing something I'd think the only reason to cut another keyway would be to reclock the dampener slit location & considering the time/money to do that (properly) I'd just get another used one as they are pretty much bulletproof (yours is the only bad one I've ever seen unfortunately).
My conversation with ATI centered (pun intended) on my use of a centrifugal supercharger overloading a single OEM keyway. The Op's broke at the keyway as best as I can tell, and is apparently the weak point that ATI was trying to beat into my brain, and it looks like they have seen this before.
