MC34063 with external FET. Why?
Apart from the problem of the PNP transistor being upside down (which I believe you have corrected), you might find that the newly corrected PNP will not switch off fast enough to deliver a decent square wave to the gate of the MOSFET. The input capacitance to the MOSFET (between gate and source) is 1.6 nF and this needs discharging probably a lot quicker than the circuit you have implemented.

If you look at the gate threshold voltage in the data sheet you will notice that it is 0.7 volts (quite low) and the MOSFET will not properly turn off until below that voltage. I suspect that trying to discharge the gate capacitor's voltage to that low level is the problem.