Does the crack go all the way from the mouting bolt to the vacuum pump sealing surface? If so, that's a heck of a crack to fix. I think it'll be cheaper to replace it. If it feels solid despite the crack then maybe cutting a 'V' into the crack on both sides and filling with JB Weld is good enough. That's a big labor investment just to see if it works, though.

Sixto
95 S420
87 300SDL