gman_modz on eBay is selling a new from-scratch FRAM based N64 Memory Pak. FRAM does not require a save battery and is a rugged replacement that shouldn’t lose saves. The Paks come with a 3D printed shell, or the PCB fits an original N64 Memory Pak as a drop-in replacement. They cost $28 + S&H.
GMANMODZ documents the process of creating the new boards on their blog.