"Contrary to common opinion" most old lenses were not soft. Look at any old daguerrotype with a strong loupe, and you'll find it to be sharper than any modern film!
But all is not lost: Portrait lenses and faster Rapid Rectilinears (and Aplanats) have a very limited sharp coverage at full aperture, maybe 15 to 30°. The center sharpness rivals any modern lens (and outperforms most), but the sharpness drops off rapidly and dramatically towards the edges. Using a RR or Aplanat in about the same focal length as the film diagonal (like a 6" on 4x5") will get you "authentic" softness for around $15.