The photos provoke more questions than answers. I see quite a bit of cold patches indicating crack "Repairs". So my first is were the cracks cut and repaired or simply smeared with "cream cheese" to hide the cracks. Look closely, cracks will reappear before long if not properly repaired. If the installation is 100 years old, We are looking at plank sheathing as compared to plywood. Plank sheathing can take more water than plywood. The amount of cracks bothers me, especially considering we are looking at a limited area. It is important to determine if the stucco is adequately adhered to the structure, or is the only thing holding the stucco to the structure; the stucco itself. I would suggest moisture content testing to determine if/how much water is getting behind the system, at the same time testing the resistance of the wood sheathing. Core tests would also let you know what is there, especially below that window. Regarding other details, sealants, etc... more information needed as well as up close examination. Structurally speaking, if you determine that all is strong and stable, that is great. Cosmetically speaking, I think the only salvation would be a fresh coat (with mesh) of stucco, or another cladding, possibly EIFS or another alternative. Included should be WRB, flashing, etc., I lean towards liquid applied.