You're correct that the hole is for alignment of the shaft, the pump driven impeller, the shaft bushing, and the gear mesh on the cam. And while the broken area is indeed thin, the direction in which it is stressed is horizontally to the right in the last picture, not outward from the axis of the pump shaft. This recess is what prevents the pump from wanting to move around the hold-down bolts in a CCW fashion (when viewed from the bottom); these bolts by themselves can't necessarily be counted on the keep the pump perfectly aligned over the long term; there is no other indexing in the setup other than the recess.
The right side of the recess in the bottom pix, towards the crank throw, takes move of the side force of the pump trying to move. The loss of the broken piece removes some of the material helping to hold that side in place. I don't think any of us can tell for sure if that right side of the recess will eventually crack out too and perhaps allow the pump to move out of alignment, or if it will stay solid for ever.
I assume you are seeking opinions. Understanding the above, I would personally want to do something to restore the integrity of the recess.
If you choose to not do anything, then I would suggest using Permatex #1 in place of a pump gasket, after cleaning both surfaces thoroughly and making sure three are no burrs on the surfaces, and letting it set up overnight. Permatex #1 is designed to oil seal 2 metal to metal surfaces, and provide a gluing action between them; it was developed to join the 2 crankcase halves of small aircraft opposed cylinder engines with an oil-tight seal, and I would think would provide some 'glue strength' to assist in keeping the pump from moving.
