If the motors are fine, the resistor would be the best idea. Make sure to mount this resistor in the engine area because it gets hot. You should be able to tie all motors to this resistor. Some 6-volt vehicles such as GM and GMC also have a positive ground. You will need to buy an inverter to keep your instruments working properly.
Inverters usually cost less than converting instruments to a negative grounding system.
To convert positive ground to negative ground, you need to reverse the wires on the ammeter and the coil. You will also need to reverse the battery cables. If you are unsure how to so this, consult your mechanic. If your vehicle has a starting solenoid for your starter, you may wish to change the solenoid to 12-volts. Most 6-volt solenoids will work on 12 volts.