In what state are you located? Each state has different policies and prices for testing and necropsy. Your own regular vet might know how you could get some swabs on your chickens yourself, and send them in for a culture. Zoologix.com will do a test for up to 8 respiratory diseases. They send you the swabs to collect, and send back. That is $90. It is possible to have more than one disease present. The fowl odor you are smelling would more likely be coryza or canker. The foamy eye is common with MG, but may be found in other sinus or respiratory diseases. If you could post pictures of the yellow plaques inside the beak, it would be helpful. Here is a link for Zoologix to contact and ask questions:
http://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
Your state poultry vet (found in the list below) could be helpful about testing as well, and also may cost less to test one or two birds:
https://www.metzerfarms.com/PoultryLabs.cfm
http://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
Your state poultry vet (found in the list below) could be helpful about testing as well, and also may cost less to test one or two birds:
https://www.metzerfarms.com/PoultryLabs.cfm