Tried to boot this up but the game cartridge needs a new ba…

Let's fix that! That screw is a little fucker to open without the correct screwdriver. I used the point of a saw on my swiss army knife to get it open. anyting > shaped should do the trick. Rightey-tightey lefty-loosy!
Old patent was inside :D

Find the correct battery. Old one was made in CHYNA!
Assemble like this:

contact
-
battery
+
contact

Added Blu-Tack for extra hold!

Sealed the cartridge, put the screw back and booted the game. Saving now works!