I think in this instance the problem would not be solved by going to an opto. The OP mentioned his points were sparking - there should be a capacitor(condensor) across the points not only to stop the sparking, but to ensure more energy gets put into the coil. Sheilding the cable from the hall effect sensor I think would be beneficial - the cars I've come across that use hall effect sensors do this. The case of the engine should also be connected to the 0V of the micro (& associated circuitry) so that capacitive coupling of the ignition pulse does not occur.

Optos have fallen out of favour in automotive applications as they age. Reluctors are cheaper and more preferred - these are simply a electromagnet with a magnetised core, whenever a metal item enters the pole gap, a positive pulse is generated and when it leaves a negative pulse is generated.