Use stranded wire intended for auto wiring.
Vibration and movement will cause solid wire to fail, it may take a while but it WILL eventual fail.
You will never find production wiring on any mobile vehicle using solid conductor wiring.
While spending the time to wire your tractor... add a few more moments and a couple dollars extra. Put relay controls on your lights.
Use power from the key switch to your light and fan switches, then have those switches activate relays to apply fused power to the lights or fan. (this will take any load away from the key switch AND will prevent an unintended accidental discharged battery from lights left on with key off.
Use the correct gage wire for the circuit, enclose wire in flex wire loom, it will look and preform like factory engineered.
Doing it right the first time will save you Time, Aggravation and Money, PLUS will reflect pride in a job well done. KennyV