Hi,
If you want to build a zapper you dont have to go through all that trouble. Many times an ordinary DC wall wart of suitable size (9v, 1amp) would work as long as it has an internal capacitor. If it doesnt or if you would like to make it even better, provide your own external large value cap like 3300uf or better.
To use it, connect the cell to the wall wart for a second or two using heavy jumper leads. That will revive some NiCd cells.
Unfortunately, these cells usually go bad again shortly after zapping anyway so it ends up being a waste of time.