This is a bit of a work in progress, but I’ve found a 10000mAh powerbank that fits inside the empty battery pack.
Amazon Link
With the relevant PD breakout board it could provide 12V either for my custom LCD replacement (see other post) or perhaps power the whole laptop?
Making good progress on this one.
iPad LCD replacement for powerbook - also boosts the resolution to 800x600!
EDIT: More details of what's going on here....
I've 3d printed a bezel for the LCD to fit the original bezel:
Excuse the fingerprints, and the colour choice of filament. I need to...