Who in their right mind hides component designators in a sim?
You don't need the .inc MC34063.lib as that's built into the symbol.
It looks like the MC34063 model has a problem. If you don't use the .tran startup or uic options the operating point analysis at the beginning of the .tran run can leave it stalled in a non-oscillating steady state. There are other models in the .lib file that may be worth trying - edit U2's value (currently MC34063) to select them.
Choosing a realistic model for the diode (e.g. 1N5819) helps with timestep problems as does setting initial conditions for the two big capacitors so the massive initial charging current surges aren't included in the sim. Label the Vcc node, then use:
.ic V(vout)=11.5V V(vcc)=12V
It then runs OK (under LTspice IV) with default settings and no.options overrides, with a simple
.tran 100m uic