On that tractor I'd probably just replace the ammeter... for the simple reason that it's easy and it keeps it original... and for the most part it gives you as much info as you need anyway.
If you wanted to use a voltmeter there isn't much rewiring to do. Just nip the ring ends from the wires that went to the ammeter and join them together with a butt connector. Then take a switched power source off the key to feed the voltmeter along with a ground for the other side of it. Done.
If you had a ground disconnect on the tractor... all I'd do is hook the two ring terminals on the present wire to the pos stud on the voltmeter, then ground the other side of the meter and call it good. A VM will discharge the battery if it remains connected when the tractor is not in use...