I want to run the headlights to run off of the battery when the engine is not running
If your lights only work when the engine is running, then you only have a 3 AMP charging system, so likely will have to use a battery charger to top off the battery.
The lights are powered off a separate AC lighting circuit.
Easiest way would be to just add an extra set OF LED light(/s) (low power) with their own switch.
Use the AC powered lights when running to conserve the battery and switch on the lower power light when not.
Factory lights would draw about 5-6 Amps, so you'd need 2 minutes of running at high engine speed for each minute of headlight use.
This is assuming you have no other electrical draw such as a fuel solenoid or Operator Presence Relays.
Actually I was thinking about doing that and adding a volt gauge but I would rather have the volt gauge so how do i in stall the gauge?
Not sure what you want to accomplish with a volt meter? Do you mean ammeter?
For a volt meter, just wire it in to the circuit that powers the carb fuel solenoid, with the other terminal to ground.
That way it's OFF when the key is OFF.
If an ammeter, it has to be in series with the charge wire between the key switch and charge diode off the alternator.
Since you didn't provide any tractor brand/model etc. one can't give much more advice than that or ensure that is the absolute best method.