Replace potentiometer with a mosfet

Hi,
I'd like to replace the potentiometer on this motor controller, motor controller , with a mosfet to be able to control it with arduino, is this possible? The pot in that controller is 1/2w and 100k Ohm.

Unlikely, as a potentiometer is a variable tapped resistor.

You need a digital potentiometer, such as this one:

You might be better off with a "digital potentiometer", but there lots of caveots to use one. Some reverse engineering of the circuit would be necessary.

What Nigel failed to mention – on the digipot he linked and in fact on many digipots, the resistance terminals are limited to the supply voltage of the chip, which is 5 volts in this case.

Without understanding the power supply circuit, this digipot probably won't work. There are a limited number of types that will handle more voltage, but you'd have to do some experimenting to figure out the range needed.