Same here when I put metal roofing on older barns that had sagged rafters.

Easiest way to make it straight again was to sister on new rafters to all the old ones. making the tops of all the new ones level to give a nice flat surface for the sheeting and eventual metal roof.

Old rafters and lathe.

Also added trusses since supporting posts were removed when a corncrib area was removed to gain wider storage space.

Doubled up rafters and sheeting

Formed on site standing seam with no exposed fasteners.