The 555 regulator that won't regulate
At first I tried to run it using 5v but could not reach a 170v output


I'm not surprised, at 5V the 555 will barely be able to turn the MOSFET on, let alone do it quickly enough. As it is, the 555 isn't great at driving a MOSFET well enough for this application, but the higher voltage gives it a better chance.


  I opted for a 12V supply and put in an LM7809 regulator...because the output voltage would vary with the input voltage and I wanted a stable output.


That variation is happening because the rest of the circuit isn't working well enough for the feedback mechanism to become active and regulate the voltage. As well as reducing the drive to the MOSFET, the 7809 will have a current limiting effect which could stop the inductor being charged properly. With the rest of the circuit working it will be pointless.


  I may have used an IN4004 diode instead of a UF4004 - is there a difference?


Yes, a big difference (components are usually specified for a reason). The UF4004 is a much faster diode than the 1N4004, which is crucial when you're building a switching supply running at about 30kHz.

Finally, the 200V rating on your output capacitor gives very little headroom. I'd use a part with a rating of at least 300V. Also pay particular attention to construction and possible arcing between the high voltage connections.