No.
The generator requires an excitation source, i.e. an electrical excitation generator or a permanent magnet generator. The electric fan cannot be upgraded if it does not use this type of motor.
It is not possible to add permanent magnets to the rotor. Motors with permanent rotor magnets are electric motors and generators are stator-excited motors.
In addition, there are many factors. Electric fan motors are usually single-phase, while generators are three-phase motors. There are also electric fan motors with brushes. Generally, generators do not use brushes. The structure needs to be improved a lot. There is a big difference between the full flow of the generator location and that of the engine.