It does not charge when running but the generator guy says it's a good generator. OK.... You have an open circuit someplace but where & how do we find it?
First question: Does the voltage regulator have a good solid ground? If not, it won't work correctly and it will not make the charging sytem "come to life". Make sure the voltage regulator it is solidly grounded. This includes the ittle braid across any rubber anti vibration mounts!
If the regulator is properly grounded and still it does not charge, I would remove both wires from the generator's posts. Then I would ground the F post to the generator's frame, NOT the tractor's frame but to the generator's frame. Then I would take a jumper from the battery's NON GROUNDED POST to the GEN or ARM post. It may be labled A, G, GEN or ARM. Have seen 'em all. When you just brush this jumper across the generator's post it should spark rather stoutly and the generator should try to run like a motor.
If it DOES, your trouble is probably an open wire between the generator and the voltage regulator or between the regulator and the ammeter or the ammeter is bad or the regulator is bad or you have an open wire between the ammeter & the battery.
If there are no sparks as you brush this jumper across the post, either the generator is open (though the generator guy says not so) or you have a bad ground connection between the generator and the block.
Next I would run a jumper from the GROUNDED post of the battery to the frame of the generator. Not to any of it's brackets but right to a spot filed shiny and bare on the generator's housing! (Fix the paint later.) Brush that OTHER jumper across the A post now & see if it sparks! If it does and the generator tries to run like a motor, remove all the jumpers, remove the generator and then remove the generator mounting bracket from the block and CLEAN ITS ELECTRICAL CONNECTION TO THE BLOCK as well as the connections between the bracket and the generator!
Hope that helps.