Is the foul odor coming from the face or just a smelly odor overall, possiblyfrom the cow barn? Coryza is said to have a bad odor, and is more serious than MG. You need to get the testing to figure out whether it is MG or coryza. Try to find Tylan or order the tylosin powder and start treatment. Sulfa antibiotics may be helpful if it is coryza. Make sure the chickens are in a coop with fresh air, no dust, and no wet or moldy conditions. Here is a link to read about MG (mycoplasma gallisepticum) and infectious coryza:
https://edis.ifas.ufl.edu/ps044