Where it is possible to retain the original fabric of the building, we would avoid replacement in new stone in favour of a repair.
This could require the pinning or stitching of stone with stainless steel bar, or perhaps colour matching a plastic (lime) repair using wire armatures where necessary. Often it is only necessary to de-frass (brush/ pick off loose material), crack-fill, and/or consolidate with a lime based mortar or shelter coat.
One of the most important roles in conserving buildings is that played by maintaining the pointing. We use a selection of sands and various strength Naturally Hydraulic Lime (NHL) to match the original mortar.