Running a MOSFET from a transistor for a fan controller
I'm taking a guess that the B772 (PNP) collector is connected to the +12V terminal of the fan and that the circuit switches this on and off as a PWM control of speed.

To replace the NPN transistor use a P Channel MOSFET. This needs to be switched ON by grounding the gate so an inverting transistor is needed.



When the B772 is turned ON a small current flows through R1 and R2  and Q1 is turned ON pulling the gate voltage to about 0.1V. This turns the MOSFET ON this supplies current for the fan. There are lots of MOSFETs out there that would be suitable.

Be careful you don't overload the power supply of the controller