You can get new guage face decals from numerous vendor's, also the fuel guage is the only guage that will have a problem with 12v instead of 6v and you can get a resistor to step down the voltage and still be able to use it.
Also if you have an orig. heater you will need a resistor for it also, the heater motor can't take the 12v pwr.
Try some of these
www.fillingstation.com
www.bowtiebits.com