Most batteries are either cobbled together from existing packs or form factors, so you can often fill the bill with something off the shelf at Radio Shack.
Crack open the battery pack (no loss, right, since it is useless anyhow?), depending on whether you have the high capacity pack or not, you'll probably see six or seven cells shrinkwrapped together inside it. Judging by the fact that the battery packs have differing output voltages, the voltage regulator in the radio is probably pretty robust.
Looking at the drawing it is probably just a bunch of AA or AAA-size rechargable NIMH batteries. You could probably just solder them together and build a new battery pack, glue it inside the old plastic case, and you'd be set.
However, what would really do it for you is
this google search for "ICOM IC M1 battery pack" where I turned up a bunch of results for purchasable packs. More expensive than homemade, but probably more dependable. (-: