I would dig out an area for a footing as close to the wall as I could with out damaging what is left of the foundation, my dig out would be for a footing 18"x 60" and 8" deep centered on the sag. Hang a peice of dry 6x6 timber from the joists over the footing. With a 20 ton jack in the center, jack it up. This may take a long time to lift it up, 1/8" per week. When it is up, put a 6x6 post at each end remove the jack and repace it with a third post.
I know, your not supposed to use a post but after it is blocked up, it is not doing much but will add stibility for the jack.
The footing will need a grid of rebar in it.