Did you replace another alternator? If that tractor had the Motorola alternator, then it has an electronic voltage regulator located beneath the floor plate up under where the stairs attach. If this is the system you have, then a single wire self energizing alternator may be in conflict with the regulator if it is still in the circuit.
On my early 8000, it had the low amp output Lucas generator and a remote (mechanical) voltage regulator. We had to rearrange some of the wiring around the voltage regulator to allow the alternator to charge the battery.
If you are replacing a single wire self exciting alternator with another one, and the first one did charge the battery when it was working, then the tractor had been rewired to bypass the regulator. The wiring and its insulation is so old and brittle that anything could be the reason for failure. I think it is going to take some snooping with a voltmeter and a limited electrical load such as an old 12V auto headlight with jumper wires attached to the headlamp. I sugest this as I have been fooled by a good voltmeter reading when the wires have corroded or broken from vibration. There may be enough of a connection to give a voltmeter reading, but not enough good copper to carry any amps (thus the need for the headlight test load of about 5 amps).
Earlier this summer, I was fooled into spending $$ on a new battery for our 9000 tractor when in fact a good looking battery cable had internal corrosion that allowed correct voltmeter readings, but no power for starting or battery charging. These problems can be very frustrating!!
Paul in MN