The heating element you have will require 69.6 volts to provide 120 watts of heating. To use a 12 volt supply you need an heating element with a resistance of 1.2 ohms. Your existing heating element has a resistance of 40.33 ohms so with a 12 volt supply would provide 12 x 12 /40.33 watts = 3.57 watts. The only way you could use the existing element would be to use a step up converter to convert the 12 volts up to about 70 volts. The converter would only be about 90% efficient so you would be using about 132 watts from the 12 vollt supply to get 120 watts of heating.
Les.