The NiCad battery pack in my ICu2AT HT wouldn't hold a charge anymore, as NiCads often fail to do... But I had a Nikon camera Li-Ion battery pack that is rated for 7.2V and charges up to 8.4V, at 1500mAH. And the rig seems quite happy running off of a Li-Ion battery, so I removed the NiCads and its charge circuit from the rig's battery case, and stuck the Li-Ion in there instead, with room to spare. Of course, I need to make use of the Li-Ion battery charger that goes with this battery, which has 3 contacts to the battery, so I have to change the battery pack's charger jack. So I can connect the 3 leads to the battery inside the Icom's battery pack. I thought of using a mini stereo jack (3 contacts) but I think the charger and battery would get unhappy with the momentary shorts one gets when plugging in and removing the stereo jack. So I used a 3 pin in parallel jack and plug, the sort used on circuit boards in consumer equipment like VCRs, and soldered inside the Li-Ion charger wire leads from this new connector.
When selecting a Li-Ion battery to use, you need to pick one that produces a supply voltage the rig will operate off of. That's obvious,
and less important but still worth looking at is the mAH rating, 50% tolerance should work fine, and of course a battery that will physically fit inside the rig's battery case. That and some solder and hot melt glue and similar ham radio construction techniques should get you back on the air.