Properly rated components for 12V -> 500V booster circuit
You want the resistance of the inductor and MOSFET to be as low as reasonable 
to minimized loss. The current should be limited by the on-time of the MOSFET (not the resistance).

From the basic relationship \$ V_L = L \frac{dI_L}{dt} \$. For constant \$ V_L \$, \$ \Delta t = \frac{L \Delta I_L}{V_L} \$.

Plug in peak current \$ \Delta I_L = 4A, V_L = 12V, L = 130uH \rightarrow \Delta t = 43us\$.

So you need to set the 555 to have the MOSFET on-time to be less than 43us (leave some margin).