Twelve volt batteries are actually 13 volts but most people use the nominal 12 when describing them. Lead acid cells are 2.15 volts per cell and a (nominal) 12 has 6 cells, which rounds to 13. (note to OP) Calling your 12 volt a 'juiced 13' will serve to invite confusion away from the answers you seek.
How to:
- reverse the battery terminals.
- reverse the ammeter wire.
- radio is a potential problem.
- if there are any motors, like heater...they must also be reversed.
- bulbs and lamps can be ignored
- reverse the ignition coil
Reverse is hopefully understood to mean changing one for the other.