Probably need a bit more info about the building, but in principle, the old ceiling beams shouldn't have anything to do with the stability of the stone walls.
Old walls tend to be quite thick (=stable, even laterally). It can depend a bit on the material the wall is built from. But if the wall is intact and sound and not noticeably leaning, it should be fine.
The newer roof should be anchored to the frame, otherwise, there's a risk it will blow off in the first good storm. Newer trusses and metal roofs are indeed flimsy and light roofs that work well as sails.