This might sound kind of cold, but - if it means nothing to him to see you keep it, well, perhaps it should mean nothing to you to let it go.
Personally, I'd have a hard time feeling sentimental about something my father didn't give a rats ()#$ about keeping in the family.
That tractor would be about as meaningful to me as any one I found on craigslist, and I'd take my kids on a nice little vacation with the money I saved.
Having said that - have you even discussed it with him? I'm assuming you have, but the way your post reads, it sounds like you haven't even expressed any interest in it to him.