A working rule of thumb. To get the same "work" at 6 Volts you need 2 times the current. The wire harness for a 6 Volt system has bigger wire than the 12 Volt harness. A 6 Volt wire harness is fine for a 12 Volt conversion. The end items need to be replaced, or have a dropping resistor to save them( Coil, needs a Ballast resistor. Lights, replace. Heater motor, replace or control the current to the speed control. Radio, dropping resistor. Gas gauge, dropping resistor or a voltage regulator.) The starter motor will work at 12 Volts just don't run it for a long time. It will get hot. That is about it.
