Need help in designing of circuitry. (Power Electronics)
You can solve this easily by using 2 timers : 
See the figure below.
The second output should be used to drive the MOSFET.

the first timer works in astable mode and sets the frequency which triggers the second timer (on the right) which is used in monostable mode. 

You did not specify anything about frequency and duty cycle range, but by inserting your LDR resistor for R4, the duty cycle will change accordingly. 

So when there is a lot of light, R4 (your light sensitive resistor) decreases and duty cycle decreases and vice versa.

The values need to tuned to your requirements for frequency and duty cycle range.