Hello,
The Motor can turn in either direction depending on which terminals i connect the power to.
Do you need to change the fan direction with this new system also??
Also, we don't know how the speed controller works. Can you carefully measure the voltage across the potentiometer to see what's needed..
The LED-driven LDR (Light dependent resistor) would be the easiest method, but the control VS LED current will be non-linear, but probably workable in software...
You need an LDR with enough power dissipation rating...