definitely a great hobby project.

i would consider building a more general purpose piggyback ignition controller.

make it accept many types of signal.. opti, various reluctors. make it work for arbitrary number of cylinders.

make it able to drive coil per cylinder, waste spark, -or- single coils for distributors.

give it a loadable config file. add two analog inputs for a map or tps.

give it tables for add/subtract advance, dwell, multiple sparks per cycle, cut and retard options for nos n rev limit....

arduino for a brain should be able to do it

if its worth doing its worth doing well