If you have some decent jump leads, you can ascertain if it is the live side where the fault lies. Might need someone to set it cranking then see if cranking speed increases when jump lead added as an extra conductor. Or just try with jump leads if a bendix starter. Just be careful, so no sparks occur anywhere near a battery. For earth side check, just add a jump lead to earth side of battery (not to the connection to the battery) and tractor frame.
It should get warm or hot at any high resistance connection, if cranked a lot, but if there are two or more poor connections, they may not. My first job, if without a meter or jump leads, would be searching for a hot spot and next would be remaking the connections, ensuring bright and tight metal to metal contact.
It may be a poor solenoid if a bendix starter.
As you have checked out the battery, there is not that much left to check, really.