Possible To Achieve 1kw With UC3842 ?

Is that possible to achieve 1kw with UC3842 ?

The answer is "Yes",  you could get 1KW out of this circuit with the following conditions met:
1) The switching FET needs to be a pretty big device (like an IRFP460) with very good heatsinking
2) The switching transformer needs to be pretty serious (like an ETD49), well designed, and close to the PCB
3) You need to add an R-C filter that includes a steering diode (see other flyback SMPS's for an idea how to implement)
4) The 300V supply needs to be quite robust (are you going to use PFC?)
5) Make sure the switching frequency is not too high to have copper losses impact the power level (like maybe 25 to 50KHz)

The small transformer you have a question-mark over looks to be a feedback and Vsupply source. During normal operation, the power FET's switching is coupled through the 22nF/250VAC cap and the transformer generates two voltages - one to supply Vcc, pin 7, of the 3842 IC and the other voltage as feedback to pin 2. It looks like the current sense is set to about 4.5A (1V/0.22ohm Source resistor), so with 300V in, you're at about 1.3KW - on target with your goal.

This looks like a great project to experiment with!