The design that Richard explains was an opportunity to improve the starting voltage. On the 12 v systems, the resistor was bypassed by the solenoid when starting, making up for some of the drain on the system voltage. I believe that you can buy 12v coils that don't need the resistor. If you are using the foot type starter, the resistor type of coil would not be of any advantage.
There are a number of types of reducers. I think that you can get 12v replacement motors for the fan but I don't know about the gas gauge.
