I need to vary / delay a voltage to an HT coil, thus delaying the spark
That sounds like a highly dubious thing to want to do to an engine in the first place - I'd question what your reason is for needing to do that before anything else.

If you must do it, you will really need a microcontroller to change the delay based on RPM as it's the angle of advance/retard that's important to the engine running.

Easiest option would be to use something existing, like MegaJolt (open-source, cheap, documented, hard work done for you).