I'm kinda lost here.  Early in your post, you said:  "he's mentioned to me that he has no need for my tractor and if I wanted to get it or sell it then I should come get it."   Then later you mention a charge.  Was this mentioned up front?  Did he change his mind?  If you would have gone and gotten it before talking to your cousin, would there have been a charge for the repairs? 

I'm surprised that he would go ahead and put money into repairing a tractor that he didn't owned, which you did not authorize to be repaired, and then expected you to reimburse him for said repairs.  

I think Cottonhauler has made a good suggestion.  If there is an hour-meter working,  make note of it.  (of course, maybe you didn't record the hours on the meter when you bought it?)