There's lots of pics and instructions online for a Sys11 battery repair. However, in a nutshell:
1) Desolder it

2) If there appears to be ANY battery damage on the board, mix half and half water and white vinegar and wash all affected areas (and a bit more) on the board. Rinse with water and dry immediately with blow drier/compressed air (blow out from under any chips). This neutralizes the leakage. Repair any damaged traces -- look close and use a resistance tester on your multimeter--should be 0.03 ohms or less. The traces are the thinnest of any boards of that era and easily damaged.

3) Brush the + and - holes (lower left towards center of board, upper right towards top edge of board) and solder in at least 18" of wire, preferably red and black colored to know which is which.

4) Buy a 3 position AA battery holder (2 for 99 cents shipped from China via eBay), a 4 position will also work if you solder a wire across one of the bays. You do NOT need to install a diode on a Williams as there is already a diode on the board. The diode prevents charging of the batteries.

5) Solder the battery holder to the two wires you already installed. Solder direct to the terminals as the ones included are usually 26 gauge crap thin and breakable.

6) Lay the pack on the bottom of the backbox, or screw it to the side. (I prefer laying on the bottom because if you ever have to remove the board for work, if you screw it to the side, you'd have to remove a battery and lose your high scores to detach it)

7) Consider buying Energizer Ultimate Lithium batteries (with a 2033 or later expiration date). They will last a lot longer before needing to be changed, and info from others is that they almost never have any leakage concerns.