Well, there are 2 things that can happen.. 3 if you count failing to prove anything.
IF the seller admits he 'forgot' or even didnt stress the problem hard enough (mumbled it..), he could be liable for repairs, or a partial refund to bring the price in line with the new value. As said, this is usually hard to prove unless you have documentation proving that you asked and the seller denied. In this case, the OP has the seller admitting 'he forgot to tell'. Thats a pretty big thing to 'forget'.
The other is IF you can prove, or the judge believes, that the seller intentionally misled the OP during the sale of the tractor in order to either sale something that an otherwise reasonable buyer would not have bought, or, to avoid having to sale the tractor at a lower price than other comparable tractors, then the OP could ask that ALL the monies involved be returned and the sale canceled. The judge also then can levy additional fines on top, depending on the law.
Of course, actually collecting on any rewards in your favor is a entirely different matter.