The interface hardware is pretty simple. My end goal is to build it around an ItsyBitsy M4 Express board. Alas there aren't any yet, so I did the next best thing: I build it as a Metro M4 Express shield. Not the compact form factor I wanted, but all the same capabilities.
There's a microSD breakout hooked to the SPI lines, a 128x32 OLED on the I2C lines, and a rotary encoder (with push switch) using 3 digital inputs.
See the related guides for more details on each of these:
Because I made the decision to use I2C between the interface circuit and the emulator circuit, any MCU with I2C capabilities would work.
One small wrinkle: the Metro M4 is a 3.3v system while the emulator is 5v. Adafruit to the rescue with the BSS138 based 4-channel I2C-safe Bi-directional Logic Level Converter. We just need to run the I2C lines through this between the two circuits.