I think, there is no need to back up the content of an MASK ROM in a board. Because the possibility of being destroyed is not higher than for any other component. Its content cannot be deleted or altered by reprogramming etc. But if you want to be sure, the ROM could be read out with any EPROM programmer that supports 40-pin-chips, and stored as a file. If there were a problem in future, you could make a new EPROM with this. Flash and EEPROM is quite expensive, but of course, you can use it as replacement. If you make your own pcb for that chip, anything like the 27C402 should work. (As far as i know, the 27C202 (1M) is not available any more).
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