I replaced the battery pack with new cells. I soldered them together (like the original) and used the power cable from the original pack. The originals were Sanyo Cadnica 1.2v 700mAh, new ones were (by surprise) also Sanyo Cadnica, this time 1100 mAh. Even higher capacity ones were available but out of stock at the time I bought them.
Has been working perfectly. The battery compartment is a bit tight so no much space for alterations.
Left: original battery pack, right: my self-made new version.
Bang & Olufsen - The art of controlling sound, picture and light