Your timing will be no more accurate than the underlying mechanical sensor, which is likely to be in pretty poor condition if it's as worn as you say. If you're going to go to the trouble of doing this, I'd suggest you look into the possibility of fitting a trigger wheel somehow.
Cars and bikes tend to be electrically quite hostile environments and at a minimum you will need some external filtering and protection on all the inputs; you will also need to be very wary of spikes on the ignition circuit. Even with commercial products designed for the job, I've had problems with electronics resetting due to noise.
What you'd end up with is essentially equivalent to a one cylinder megajolt. Have you considered just making one of those, and taking advantage of the existing hardware and software designs?
If you're anywhere near burning holes in pistons and it's an engine you care about (and assuming it's basically sound) then you really ought to be looking at monitoring and controlling the air/fuel ratio in conjunction with the ignition, and monitoring for knock.
Your other option is to remanufacture the existing mechanical dizzy, which would probably be a much quicker and cheaper (and safer) option and would make sense if it's just an old engine that you want to keep running without doing any further damage.