I would strongly advise being very, very cautious before providing a capacity. There are far too many unknowns for construction of this type and that age. To the extent guides exist, they are more of rules of thumb that require pickup trucks full of salt. When I have dealt with similar situations in the past I always relied on a newly constructed interior foundation system. Maybe that's conservative but I feel it prudent given the large unknowns.

Here is a structure article on old retaining walls. Not the same but gives you a feel for how uncertain everything about old stone walls can be.

Thank you Enable! I agree. There is obviously "some" capacity left but the cost of quantifying that capacity would outweigh the cost to install a properly designed concrete foundation.

About contacting foundation contractors that deal with old stone wall foundations. I don't know where about you are making this inquiry but I remember some twenty years ago a foundation contractor was refurbishing a house that was around one hundred year old in Hadley Massachusetts. New England has a fair number of houses built in the late 1800's and early 1900's so such upgrade is not uncommon. The original stone foundation of that house was upgraded however the details of that upgrade escapes me but I do remember that new concrete was poured during that upgrade.