RAM chip
MVS RAM chips often fry due to overvoltage from a badly adjusted or failing cab power supply. It appears that the most common failure mode is the inability to write new data, resulting in errors during system self-test at the beginning of address ranges.
Basically, all NeoGeo RAM chips with "256" or "257" in their markings are 8-bit 32kB RAMs.
Pinout and cross-refence
To do.
Speed grade
See memory speed.
Replacement
Look for 5 volt, 32kB (32K x 8bit) / 256kbit SRAM (Static RAM) with the right speed grade (most today are 45ns, so any will do) in 28 pin SOIC package. Cypress, ISSI and Lyontek still make them.