This isn't likely the expected answer but it might be the best. I read it somewhere, I forget where..
Set a tiny 6" cell at 90° to the main panel, and have it directly drive the motor. When the panel is full on to the sun, the small cell will be edge on, so the motor will stop. It tracks perfectly because the small cell has a shape that is inherently precise in response to light from one direction. I never saw, or figured out, the detail of how it gets aligned right for the next day, but the idea is surely to eliminate complex circuitry and keep it as simple as possible so it needs no maintenance. It might be possible to add a small mirror or something to prompt it to return to start when the sun appears next day.