

Wear gloves and eye protection. Remove that battery and dispose of it in accordance with your local laws.
Gently scrub off and any residual acid with white vinegar and a toothbrush. Brush away or blow off loosened residue until it’s gone, just be aware it’s acid and where you are are sending the particles.
Let it dry. If the rest of the board needs further cleaning you can use isopropyl alcohol to finish it off.
After it’s clean make sure to check for damage before you replace the battery. It’s likely fine, but if it’s been sitting in acid for a while it never hurts to give the board a look over for shorts, cracks, or solder points that lost contact. If it looks good replace the battery and see if she works.
If you decide to disassemble your board to make cleaning easier, I suggest taking a few pictures first. The old MBs don’t have helper notches to ensure parts are placed in the proper orientation and documentation may be hard to come by.

You’re welcome! Wow I forgot about those batteries, they were the worst! That’s an awesome board. I hope you are able to repair it.