Before replacing anything, do comprehensive checks with a voltmeter to determine where the voltage drop is occuring. Us the scientific method, develop a hypothesis, then test that hypothesis.
Remember this, If your tractor won't start, the very LAST thing the problem is likely to be is the starter. Or for that matter even the starter solenoid. There are MANY other reasons for a no start (actually a no crank) situation than a starter. The starter is the LAST place to look from a probability standpoint. Cabling and connections are the FIRST place to look. Even before battery failure. Although it only takes a few seconds with a voltmeter and an assistant to rule out or in the battery as the failure point.