You want to be flashing the original 2015 firmware, which you can find here: https://github.com/pimoroni/Picade-Sketch/tree/master/update
The actual update file you want is: https://github.com/pimoroni/Picade-Sketch/blob/master/update/other/picade_original.hex
So you should run:
./update other/picade_original.hex