The building has been this way for 50 years. what problem are you
trying to fix? is it settling? why would it start in the next 50
years? you can't bring the building up to code (lack of footings is
just one item in a long list). besides, even if you did, your return
at resale time would be pennies on the dollar.

if it was mine, i'd stand up a form board and cover the exposed sand
with some concrete. provided the foundation is relatively plumb,
level, and square and not moving, spend your money on windows,
insulation, siding, kitchens, baths. might as well flush some money
down the toilet as retrofitting footings on a perfectly good foundation.