Perhaps the entire circuit would need to be re-designed which is beyond my capabilities..
Greatly appreciated.. Jeff
It seems you want to drive an ignition coil with a 555 timer for setting the duty cycle and frequency.
The IGBT was already pointed out and will most likely be the easiest way for implementation.
I would suggest the Fairchild ignition IGBT in SMD housing.
Essentially you can drive the IGBT directly with the output of the 555, making your circuit very simple.
But then, it depends all on what you want to achieve with the circuit.
If you don't want to post the details in public domain, you can send me a PM.