So..... did you get it changed? Sounds like you did.... but you were talking about two tractors in the last post and I got lost..
How to keep mice out.... if you figure it out, there will be lots of other who want to know too. My suggestion, keep it in an area without mice. Put poison out, or get a cat. Also, keep the motor blown out, the machine clean, and start it often. Seems mice like things that don't run on a regular basis. If you keep it outside, your out of luck.