Tendon transfer. You can substitute part of the pectorals major muscle for the loss of the serratus anterior muscle. You should wait 18 months to make sure that the sa does not get better on it's own.
Diagnosis? There are many and varied reasons for a winged scapula. In our practice the vast majority of patients have scapular winging secondary to shoulder instability so it is important to have a comprehensive evaluation to determine the root cause of the winging before moving ahead with treatment.