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, JEDEC pin compatible, 32kB (32K x 8bit) / 256kbit SRAM (Static RAM) with the right speed grade (120ns or faster speed) in 28 pin SOIC package (not SOJ).
Cypress, ISSI, UMC and Lyontek still make them and most today are 45ns, so any will do. This includes 62256, 43256, cxk58257, and others.