Lithium batteries are the way to go. You can get them in bulk from Amazon, I've been using these:

https://www.amazon.com/EBL-Batteries-1-5Volt-Lithium-16/dp/B06Y5SDBFT/ref=sr_1_14

NVRAM is great if you can install it, but WPC games require desoldering and removing a chip and then soldering in a new one, without damaging or lifting any traces on these old boards. That is a feat unless you've got a lot of board soldering experience, no matter how many Youtube videos you watch about it!

Right now it's more important to get those gooey batteries and socketed chips off of the board, dunk it in a container of white vinegar, and use a toothbrush to get all of that alkaline substance neutralized and removed. Then a good rinse, overnight drip dry, and reinstall. A can of compressed air will let you get water out from in and under the components to dry faster.