You never answered my question, how were you powering the regulator? What voltage does the transformer supply, and at what current? Again, ignition coils really need 5 amps or more. This means that your transformer must be able to supply 5 amps or higher, and your rectifier diodes will have to be rated for the high current as well.
Two 1N4007s in series is only going to drop about 1.4 volts.
If you had the ignition coil connected before the regulator, as in the schematic, then you should be fine. In that schematic, the regulator is only used to supply the 555, not to power the ignition coil. If your regulators keep burning, check your wiring and connections. Make sure you have the pinout correct. Also don't forget the caps on the input and output of the 7812. They're often needed to ensure the output is stable.
In order to change the frequency you should change R2 to a potentiometer. That, based on the value of C2, will give you a range of frequencies you can go through by adjusting the potentiometer. Use a higher value for C2 to decrease the frequencies in the range, or use a lower value for C2 to increase the frequencies in the range.