The solar photopopper replaced the two photodiodes of the bare bones photovore circuit with the solar panels, by taking a reference voltage from their mid-point. This technique was simple to apply to the bare bones head circuit too (with the modification suggested by Wilf Rigter here).
The result is a dual solar panel head, with no additional photodetectors.
Like my bare bones head, this robot uses an Omron floppy drive eject motor, with a base made from hard drive spindle motor parts. The circuit is switched by a Miller Solar Engine, and there are two 3300µF storage capacitors under the solar panels. I used some salvaged surface mount components for the solar engine capacitor and diode, which make the SE nice and small, as shown in the second photograph. The chip is a 74AC240. A schematic is available.