I am sorry that I do not have a ready-made design for the circuitry. But this is a relatively simple "wheel" to invent. I would start with suitable RAM chips, either NVRAM or (which I personally would prefer) ordinary SRAM.
To make the basic logic towards the target system should be relatively straight forward, a little depending how fast this emulator must be.
To control and write it all, one may indeed use a microprocessor wth a lot of IO, but maybe that is not necessary--there are other ways to do it:
1) One may use a small amount of logic for building an address register and a data register, which is loaded by the controlling processor. thus reducting the port count very much. Even a tiny PIC would be enough. The logic might be built into a FPGA or CPLD.
2) Simplifying the whole even more, eliminate the processor completely. By using for example PC parallel port one can easily build an interface which is controlled by the PC without any additional processor. This needs some design effort, but by using a FPGA or CPLD the control logic may fit to one neat package, and most design errors may be fixed without a soldering iron.
Good luck,
Ted