If you do not have a control circuit do not operate this circuit above 6 volts.
You will burn out Q1 & Q2 if the circuit is operated for more than five minutes. You can try using a large heat sink but I have not yet tried this. Let me know if this suggestion works or not. Running this circuit at 12 volts outputs a nice spark.
If you want to run this circuit on a 12 volt car battery then you might try substituting a 500 ohm resistor in place of the 270 ohm resistor shown. This should cut down on the feedback current to about half. Therefore, you should be able to run the circuit without overheating Q1 & Q2. I have not tried this modification yet myself… happy experimenting.