FIrst thing you need to know, is what type of battery it is. If this is some kind of power tool, then it will most likely run off a Sub C Ni-mh cell. These are readily available. Try doing a google on vapex. They should be able to supply you with the cells, and you should easily be able to solder up a pack and fit it into the existing case.

If you cant do it, give me a shout, and I can talk you through it, or make up something for you.

Garry