I did something that worked out so I thought I might share this with others. I had one good and one bad battery for my 1989 TZ250. I needed another.

I could have bought two newer style batteries and a charger but I decided to put new ni-cads in the old pack. I took it apart and sure enough there were ten standard 1.2 volt batteries. The physical size was not common but available. Since they were the same size I figured they were about the same capacity. I bought the ten little batteries, soldered them together and wrapped the bundle in tape so it looks similar to the original. They took a charge from the factory Yamaha charger and put out like the other working battery pack. I have yet to cycle them but all looks good.