Hi all, not quite in agreement here. As they look due for the bin it is no harm to strip them down and see what you have. First check that the plug connection to the end cells is good, not a corroded joint usually found on the negative pole.
Get them down to the almost bare cell level and then put each battery on charge and use a volt meter to measure the voltage across each cell. If it is zero discard the cell, if it is over 2 volts discard the cell. You are looking for cells that have a reading of 1.4 -1.65 volts on charge, they should be OK.
If you have sufficient good cells and there is no reason why not, as these were originally good quality cells, you can make up a new pack.
I brought back to life a 12 volt pack by replacing 2 shorted cells some years ago (same brand). Also consider 4 and 5 cell packs, not every function needs a 6 cell pack.
If just one cell has failed then buy a new cell to replace the failed one. Keep to the same amp hour capacity and also if you replace the Tamiya connector make sure it is correctly wired. I solder with wire to connect cells together using a 70 watt iron. Clean cell surface as it will have been welded use a solder flux to tin the cell and solder the wire on quickly, I have wet tissue on hand to cool it down asap. Take some care with charged cells as they can discharge high currents and may cause heating or even a small fire.
If you strip the outer plastic coverings carefully you can re-use the casing with some adhesive tape. I have in use lots of batteries over 10 years old, the capacity slowly reduces over time and I have found using a watt meter that some very old batteries are down about 10 - 20 % on capacity, but I use relatively low powered motors and this is not a problem.
Hope this helps,
regards Roy