One word of caution about the old linoleum that's on the floor: It might contain asbestos. Better to cover it than to remove it, which should only be done by a specialist is asbestos removal. If you cover it, no problem.

Sagging floors are a common problem in older houses. I hope when you jacked it up you did so slowly, or you might cause breakage throughout the house due to the stress. Leveling a sagging floor involves putting in a new subfloor and, with some clever carpentering, propping up the new subfloor where the old one sags. I think a good carpenter with experience in this kind of thing can do it. For a DIY, it's a challenge. Good luck.

Theres another post on GW here - Remodeling? where a roofer didn't treat a skylight hole properly and the new roof looked a little dippy - people had suggestions on how to get in the attic and try to raise it up from below a little with some support "joisting" (guessing on the word there.
Check it out.