First of course, open up the thing and remove the old battery, right the new pair of batteries:
I digged through a lot of sources until I find the above 2 batteries (a 380mAh and a 270mAh), which (very tight) fit like this:
(topview. Yes the contacts o
f the bigger one stick out, but are to bendinside. sidevies: how the smaller battery gives space for the USB port):
I thought, if I could use a bit force it should fit in a new case using the whole area of the PCb and only 3mm in hight.