How much output current do you need? Be aware that the input current needed in a voltage boost circuit is greater than the output current by the same ratio as the voltage conversion.
For example: If you need 100mA out at 12 Volts, the input current needed at 1.5Volts will be at least 800mA. And it'll likely be much more due to inefficiency of the converter. And, if you could get the 555 to run at 1.5V, the circuit you show is very inefficient since you are regulating the output with a zener diode, which is very wasteful of energy.