I was into the pharmacy yesterday and spoke to one of our pharmacists who is qualified in dispensing prescription drugs for animals. We went over several topical creams that were being used in the area for ringworm in dogs and farm animals. He did a little further checking and found that the compositions in the medications could possibly be detrimental to the cats health so he would not recommend them.
Today I had reason to go to the Co-operative Feed Store, and told them of the problem with the ringworm that you are having. They pulled a bottle of M -T-G off a shelf, and said that they had a lot of customers in the farm area that are using it and found it was working quite well for ringworm, and that it was safe for cats as well.
Here's the website for Shapley's Equestrian Grooming Products check it out and see if this product would be of value to you.