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).