i actually did this. (Of course it does not fit, i filed the chip down to the pcb (and some of it) and it still works fine and then fits in the small ebike spindle. but the electronics of the motor interfere with it so much so that it will make it hang requiring a full restart before it will...
They are 2x standard 42 x 52 x 7 mm 45/45 Headset bearings. Trash quality so definitely recommend to replace those with stainless ones to save alot of energy down the line.
You posted a picture with 8 27100 cells roughly fitting in there. Maybe its somehow possible. might 3d print the measured interior to see if it can fit and then some cables squeezed next to the bottle holder buldge. If its not possible then maybe 800wh is doable. Im thinking about 48...