Not the answer you want, but see if those yards carry type O. With stone it’s not as terrible as with old brick. Your stone is going to be harder than the old clay bricks that will be demolished by hard mortars. The problem you will face is the old mortar is now the extra weak point, and failure will be exacerbated in the areas that are good now. But push comes to shove, a crumbling and hole filled wall is worse than one that might start developing cracks down the road. Masonry failure is not a linear problem but an exponential one.