Treyz,
The starter motor can draw a huge amount of current which would require a resistor or load of a giant size. The drill would be to continue to use the 6 volt starter, which should last for a very long time, or switch to a 12 volt starter.
Because using the original 6 volt starter is such a easy solution, I don't think anyone has ever found a need to develop a voltage drop for the starter.
Stuart
