Resistor on engine to increase voltage to smoke unit?

This task is usually performed by the diode-dropping method rather than a resistor.  It's been written up in dozens of OGR threads and shouldn't be hard to find with a Search.  A ~50 cent bridge rectifier has 4 diodes in it, so using 2 bridge rectifiers or 4-pairs of diodes will drop the voltage to the motor by a bit over 2.5V, raising the smoke voltage by the same. 

A suitably chosen resistor will do the same, and will consume the same amount of power and occupy roughly the same volume/space, but it's generally easier to select suitable diodes than a suitable resistor.   The diode/bridge-rectifier method will probably be cheaper since they are more readily available.  A single "power" resistor would have the advantage of only having two connections whereas the diode method requires some component interconnection as per photo.