It's worth remembering that there are alternatives to stone replacement.
Where the aim is to improve cosmetic conditions of a stone, replacement isn't always the most economical approach.
Although theres usually more to consider overall, heres an example of a repair to detail above a door inside a portico. The entrance had a really nice rope detail which had suffered on the underside of the lintel where a failed lead work on the portico roof had caused dampens and salt migration resulting accelerating stone errossion despite being internal. This needed resolving before any "patch repairs" to the stonework could be carried out.
By using a colour matched lime based plastic repair mortar designed for such repairs it was possible to avoid undermining the integrity of the lintel, reinstate the detail and lessen the visual impact of the decayed masonry.