[Haggis]

I've built over 50 for resale, and replaced about a dozen of these batteries in the past year or so and offer the following:
FWIW, my service manual and owners manual specify 50mAH for the battery. I can still find NiCad cells (#N65BC - 65 mAH rated), but prefer to use NiMH cells today (#60H - offer ~30% more capacity in the same physical size, and are more "environmentally friendly"). If you can to find them locally, be sure they have solder tags spot welded to both ends. Any decent battery shop will spot weld 2 cells together for you - or you can solder the tags together to make a 2.44 volt series battery. A little shrink wrap will hold them together nicely. I had about a 25% success rate trying to solder to the case :-). An great alternative is to use #B40H NiMH cells. These are slightly smaller and have about as much capacity as the original NiCads. You'll likely find they are a lot easier to install, given the cramped space in the case (use a little foam rubber to pack them after installation to stop them flopping around) and will run for several weeks of typical useage before needing a top up charge.
I've found corrosion (hair thin wires) to be the main problem when installing batteries on the icr-120s - nowhere near as bad on the icr-100s.
Drop me an email if I can be of any more help.
Happy New Year. Haggis.