story identification - Identify children's book from 80's

This sounds like The Werefox (original title Pure Magic), by Elizabeth Coatsworth, published in 1975.

I haven't read it myself, but it is described as a book about a boy and his friend, who is a werefox, having adventures at night (when his friend becomes a fox). I haven't located any explicit mention of encountering a combine, but it certainly seems like it could be plausible given the story's setting on an English farm.