I have done a bunch of new foundations under barns and houses. It's
not that big a deal, as long as you have lots of jacks. I'm not clear
on the configuration of the framing, but it sounds sort of like you
need to cut any rot off, make the walls the same height above the rot,
and attach a honkin' big mud sill, like a 6x6 treated timber to the
bottom of the wall. Then you can put several jacks under the mud sill
and bring the whole building up as a unit. Avoid jacking much on one
point, or you will rack the framing, possibly beyond repair.

Once you have it jacked into position, crawl around inside and re-nail
or metal strap anything that looks loose. You would hate to do all
that work, only to have the barn blow over in the next wind storm.

Hmmm. 30x70 quonset hut on stone foundation with one side swinging
free. I STILL can't get a picture of this in my head.