Tim,

I would either buy a replacement pack with similar contacts/geometry and
modify it slightly, or rebuild the pack using new cells.
mixing old and new cells is a bad idea.

as people noted, the charger makes the difference. simple timer-based
chargers are not the best. proper chargers monitor the change of current
flowing through the cell to detect the point of optimum and usually have
two modes for charging, fast and slow. for NiCd and NiMH the slow mode
saves the battery life a little bit. I used MAX charge controllers
before and they are relatively easy to plug into an existing charger.

depending on what type of charger you got, you can replace the cells
with any cells of the same geometry. if the charger is smart and
IC-based and the IC can charge NiMH, you can even change from NiCd to
NiMH. again, in this case you can put a higher capacity cells there.
if the charger is dumb, however and simply works on a timeout, you
either have to adjust the RC circuit to increase the time, or stick with
the same capacity.

hope this helps.

/ijon