I think most restorers would cut that toe completely off following the angle of the grain, and then make a new piece. The critical part is getting a tight seam so it is a good idea to have a big enough piece of wood to allow for a mistake or two. Once the seam is tight, the wood can be mated to the buttplate and cut down to match the contour of the old stock.
I agree that if possible black walnut should be used for repair of those old stocks, but if none is available, regular (Engish) walnut can be used and stained appropriately. Matching the grain is important, but again grain can be put in, courtesy of a magic marker.