I've looked at this board and there are eight PAL chips, one of which is registered, which I feel is too much effort to reverse engineer for 4 MByte of DRAM and a SCSI interface. It could be largely replaced by eight 512K byte SRAM chips. 

It uses 256k x 4-bit DRAM, which you can add four at a time. The kind of applications that require a lot of RAM are better served by a modern PC with at least a thousand times more RAM (over 4 GB) and faster with it.

I read that the SCSI interface is faster to use directly than asking an 8 MHz Z80 slave to do the work. But then it is probably simpler, cheaper and faster to bit-bang an SD card interface. The main reason for a SCSI interface is to extract data from SCSI drives, which are rare and expensive these days.