Im very sorry about this...
Could be a few diseases. Avian influenza, Mareks, Newcastle, avian cholera even has respiratory signs.
In the case of it being avian flu then antivirals like Oseltamivir or Peramivir will be needed
If avian cholera, then sulfadimethoxine, tetracyclines, erythromycin, or penicillin.
Im very sorry to say but there is no cure for Marek's or Newcastle's... it is a case of antibiotic therapy to reduce risk of further complications by secondary infection.
Blisters on chest, scaly combs also help to differentiate. Discharge from eyes or spotted combs, blindness, odd shaped pupils also helps in differentiation.
I think oxytetra is a good option at this point with Tylosin also being still a favorite medication.
Maintain fluids as possible, separate the sick and lame because this will stop them from being exposed to further harm.
Msy also help in identifying the disease:
https://edis.ifas.ufl.edu/publication/PS044