Yes. Put the transistor in the negative lead of the motor instead of the positive lead if you have both leads available. The equivalent of an NPN transistor is an N-Channel MOSFET.

In this case, the gate can be driven directly by your microcontroller pin. Be sure to add logic to ensure that the motor will be off when the system goes to reset. The simplest way is to invert the output of the 8052 with an inverter (74HC04).

I don't have access to a drawing program, but the circuit is simple enough to describe:

1. Connect the positive lead of the motor to the 30-50V supply.

2. Connect the negative lead of the motor to the MOSFET Drain.

4. Connect the MOSFET Source terminal to ground.

5. Connect the MOSFET Gate (after an inverter) to your timer output pin.

6. Because of leakage inductance, you need to connnect a reverse-biased diode from the negative motor terminal to the positive supply. Connect it so that current only flows if the negative motor lead is higher than the power supply. This will only happen because of inductive kick when you turn off the MOSFET.

And there you have it, one PWM motor drive.

I would recommend a copy of Horowitz and Hill, "The Art of Electronics" for learning about MOSFETs (and any other component). It is the best book I've ever found on the subject of electronic design.