Any old house I've every worked on has at least a few of these problum
joist are under sized for the span, there spaced more then 16" apart, fungus damaged, insect infested, split or cracked.
Everyone is fixed on a case by case by basis.
Most if not all of the fixes are done below the floor not on top.
Most will need at least need more footings and a beam ran down between the piers that are in there now to lift and spread out the load.