Most things here tend to be external.
Some have paralleled up batteries (not sure if they are internally or externally managed, individually managed is a good plan though) to give more fun.
As far as a drop in, or drop in with some light scalpel work to make up for the size difference, or drop in and attach this specially made panel on the back. Not seen anything, much less anything good (large capacity replacement batteries from deepest, darkest China tend to be less good than advertised either by no reaching capacity or said capacity lasting 5 charge cycles and a bit firebally at worst).