a) he asked you to come and get your tractor.  I would do that - go and get your tractor.

b) I would not get into an arguing match with him. 

c) From what you have posted, I would guess that you have lost value in this tractor.  (Maybe not, due to the age of the tractor?).   If you have lost value, chalk it up to experience. 

d) If he demands payment for repairs, I would not do that.  He has asked you to come and get your tractor - I would do exactly that, and not create any more conflict than you possible can.