John:
I am not sure about muffler screws for the 25 FP but I do know that the muffler screws for the Enya 25 are M3 x 25. I would not be terribly surprised if they were the same. Take your muffler with you to your local hardware store and give one a try. Also, my hobby shop has a good Dubro display of metric parts and I found M3 x 30s there.