I used NVRAM for mine, although the RAM on mine was soldered to the board which meant a little more work.  I had to desolder that first and then solder on an IC socket to attach the NVRAM.  Whoever you purchase your NVRAM from will usually include an IC socket with the purchase once they know what sort of game it's for.

I'll find out when I finally boot the game up if I did a good soldering job or whether I damaged any tracks etc hahaha