West System is really good about shelf life. I've had some two year old stuff that works just fine although the hardener was terribly discolored. With epoxy, however, you need to be sure that it is good room temperature before mixing though - I've had some incomplete cures because it was cold when I mixed.

As for the repair, I would stay away from 5200 because it will remain flexible and allow the crack to propagate. I would open the crack slightly, mix up some slow cure West System, heat the mixture to reduce the viscosity and warm the rudder up (heat gun but sunshine might work), pour into the crack, remove screw drive, repeatedly squeeze the crack to work in the epoxy, and clamp. I bet it would hold fine.

If you really want to get fancy, and depending on the shape/nature of the crack, I would seal up the crack and apply vacuum to suck in the epoxy.