My recollection of those old mixer motors and similar mechanisms is that they often shunted a resistor into the circuit, rather than interrupting the current to the motor, and this could be difficult to duplicate with a starter motor.
But if what you need is simply to engage the starter motor when the crank is turned at a certain speed, I think a governor might work if it is designed to operate a lever or rod to a switch with a detent, and with some slack in the return direction of the lever. What would happen then is that the starter would operate continuously within a preset RPM range, rather than flicking on and off at a preset speed. Many tractor engine governors use adjustable spring stops on the rod to allow for some differential.
Somewhere in my vast collection of useless things of interest, I believe I have a flyball governor, which is not at all brassy and elegant (in fact, it's a rather coarse and cheap one, made for some internal combustion application), but it might be of about the right size and configuration. I'll see if I can dig it out in the next few days, and get pictures and dimensions. It might make a good pattern.