Running the earth back to the battery is unnecessary, provided you choose a suitable earth location near to the pump.

After you've made a new earth point, get out your multimeter and test the resistance between the pump-end of the new earth wire and somewhere else on the shell, and I'll bet the resistance is negligible (just the resistance of the meter leads). If it isn't, sand down the paint at the new earth point to improve the contact.

If you do decide for some reason that you need to run the earth back to the battery, then you are doubling the length of the circuit, and thus doubling the voltage drop in the wire, and must upgauge the wire accordingly - see the pic below (it's an Excel capture from a document I put together after being concerned by some of the wiring advice given on this site). The pic is for a circuit length of 8m (4m from battery '+' via a relay to the pump, 4m back to earth) and a maximum current of 18amps (which is the max current draw of a 044 pump).