Agreed - many times failures listed in the self tests can be misleading.... But this is just a RAM chip. Nothing particularly special about it - it's a 256k SRAM. Should not be hard to find. In fact, a quick search turned up there :
http://www.ebay.com/itm/84256A-70LL-SK-INTEGRATED-CIRCUIT-IC-CHIPS-/190495604683
The difference in the part number is that yours are 100ns, these are 70ns - so, faster.
If you have a replacement chip, a quick way to test this is usually to piggyback the new chip on top of the old chip, making good contact with all the pins. If that allows the game to pass it's self test, then it's definitely the problem. If it doesn't work, it's not conclusive, because the chip on the board can fail in such a way that the paralleled RAM won't help, but it's still fun to try before sitting down to change out a chip.
Of course, start with the obvious stuff, make sure you reseat all the socketed chips on the board...
-Ian