I agree with Wyorp Rock. Penicillin does not treat most respiratory diseases. Tylan 50 can be given orally in chickens instead of injecting it. Dosage is 0.25 ml per pound given 3 times a day for 5 days. Your state vet may be helpful with testing, and Zoologix can send you materials to test yourself, but may be more expensive. Here is a link to contacting the state vet:
https://www.metzerfarms.com/poultry-labs.html
Here is the link for the respiratory panel testing:
https://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
https://www.metzerfarms.com/poultry-labs.html
Here is the link for the respiratory panel testing:
https://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm