PAL 16L8 is a programmable logic array part, you have no idea what these things are doing.
Most likely pulling code from the 27C256 and acting as a bit slice processor.
It'd be easier to strip everything off that was controlling the array drivers such as
twin Darlington Pair chips that drive the IRFZ34
and use the Mega to provide the control signals.
Or, pull the EPROM and the PALs, try to download their code out & reverse engineer it.
EPROM not so hard, just enable CE & WE and cycle thru all the address bits.
PALs, I don't know, never tired reading one back without a PAL programmer.