And replaced it with a built up section of PT lumber.
The job was quite easy as the corner post takes all the weight of the wall so no jacking required like a stick frame.
Also we discovered the source of the rot, one of the granite slabs had a raised lip extending beyond the siding channeling water against the sill. We ground that off.
The rest of the sill beams are still in great shape even after 200 years of direct contact with masonry. PT lumber has nothing on 200 year old chestnut!