What is the absolute minimum number of sensors to run a 4 stroke engine?

Good point. I was thinking of re-purposing the engine. Perhaps using it on a sawmill. All of the sensors except thottle position would work the same as before. I would just have to add to the wiring to get them to reach the new geometry.

The Grassroots Motorsports hobbyists use what’s called a “Megasquirt” in this application. It’s a few hundred dollars.