I would find peak HP speed for you tractor at rpm first. If its lower than RPM required for the Genny then use pulleys and belt drive to get the gennys required rpm. You could also look for a used Reeves drive in the required HP range. Be cautioned in higher HP and torque values reeves drives used segmented belts. They are noisy as all get out and could easily kill someone if they came apart unguarded.