What is the voltage reading out of the regulator?
What is the voltage reading into the regulator?
The ammeter will only really deflect much if the battery is low. The closer to full charge you are the less current you will have due to internal resistance of the battery.
First try starting the tractor, then removing the ground connection from the battery. If the tractor still runs, the charging system is OK. Next, read the voltage across the battery with the tractor running. It should be 7 to 8 volts. If it is, the charging system is working.