It's not clear to me what you are looking for. Generally, if you open one of these you'll find a bunch of cells that have been spot welded to each other, probably with some kind of frame. To fix it, you'd need to break those welds, test cycle each cell and replace those that are bad, or better, replace them all as that will produce a more balanced pack.
You'd need to see what cells they are using and replace with cells that can match the performance (particularly in terms of discharge rate). It's also not particularly easy to solder in replacement cells, and the heat from soldering is going to impact the cell performance (from minor to horrible). It is better to spot-weld again, but that's a price impediment to be sure.
If you have no experience with this, you need to do some learning. Li-Ion cells are quite dangerous when mishandled. A shorted cell or pack can lead to runaway thermal issues that produce a fire that is very, very difficult to stop.