how many cyl is the car ?... because using a single magnet / pickup is dangerous, - if it missfires once, then you might blow the engine.
safer too use one pickup for each cyl
i have same issue with a homemade electric motor, - which has 4 stages of power, - north,off,south,off etc etc ... and i use a 4017 decade counter... worth a look if your not familiar, - good for a single hall effect trigger system.... but as i say, if you can have one per cyl it will be safer.