dh0765,
Since the fuse blows when you try to crank the engine, you're looking for wires/devices that are energized when the key is in the 'start' position, but aren't energized when the key is in the 'on' position.
When the key is in that "start cranking" position, you're sending current through the starter solenoid which clicks (to engage the starter drive gear with the teeth on the flywheel), and then sends current to the starter motor.
When you turn the key on over to the "start cranking" position, do you hear the starter solenoid click or attempt to engage? If so, then you may have a problem with the starter. You could disconnect the wire to the solenoid (the light gauge wire, not the big heavy one) and see if the fuse still blows.
You're probably also sending current to the glow plugs, probably through a glow plug timer unit. That would be something else to check into.
My information is pretty general because I'm not familiar with your tractor, but I hope it gives you a place to start.