The automotive systems that control injector operation/timing, ignition timing, knock-sensing etc include software in what would be (in an aviation application) a safety critical branches of the fault tree. The cost of achieving and demonstrating that the branch is ALARP (let alone the 1 in 10^9 level) is prohibitive as Porsche found out with the Flugmotor. Also, as noted by others, these systems require electrical power to operate. That would necessitate re-engineering the electrical power systems on the aeroplane to the standard required for a safety-critical system because at present the electrical power systems are not designed to a "safety critical" integrity level. That's a big-ticket item in cost terms, and has further knock-ons in increased maintenance costs (as it would introduce maintenance requirements to verify the integrity of these systems is sustained).
The final question is simply one of "why would you want to?". In return for a large capital outlay and increased maintenance costs you would get a small improvement in fuel consumption and a claimed reduction in the risk of in-flight failure. In my ~250 hours of recreational flying the only engine failures I suffered were pilot-induced. None of the recreational flyers I knew had suffered any either. Don't get me wrong - thse failures happen, but non-pilot-induced ones are really rather rare.
PDR