Loose (i.e. not 'caged') bearings in the BB, in and of themselves, shouldn't be a problem.
Velobase say 11x 1/4" bearing balls per side for an Athena BB of that era:
VeloBase.com - Component: Campagnolo D0H0, Athena
You can seek info for other models there if that's not what you have.
Does the BB show any notable wear (run a fingernail over the bearing contact surfaces; visually inspection of same)?
Only so many variables there (completeness, condition, adjustment). If it continues to give you trouble, easy enough to replace w/ a low-maintenance cartridge unit...
Why replace the monoplanar calipers? Have problems with them? I would tend to think that the most appropriate levers would be the C-Record era Athena/Chorus/Record levers (either aero or non)...but that same logic would apply all around, from my own point of view.