Hi,
some time ago i remember someone here to talk about a circuit to read the audio from a tape into the ZX Spectrum
Well i am trying to do something similar:
I bought an small cricuit that has a sdcard slot and a 2.5 output jack. It reads mp3/wav files. The price is +/- 5 euros.
I am using the Arduino, i am using the UNO 3, but it can be done with the smaller one, the Nano. I connect the mp3 board to the Arduino, put a mp3 in the scard, and with some wire and programming i was able to play the mp3 and listen in headphones. It needs a 5v but with the Arduino nano it can be done with 3v. I will now put a LCD screen to see the name of the files in the sdcard, and some buttons to choose the file and play it.
Putting the ZX game file in mp3 or wav and connect the Arduino into the ZX, i think i can create a player for the ZX with a size smaller then a credit card. Total production cost should be around 20 euros.

Arduino nano +/- 6 euros
LCD +/- 7 euros
MP3 card +/- 5 euros
Buttons +/- 2 euros
wire+solder +/- 50 cents

The lcd needs a lot of iron solder to put all cables connected to the arduino.
The game will take the same time to load, but you can have all games in the sdcard. It is the same concept to the Amiga HXC, but simple to do it.
I am sure someone did this before, but it's quite fun to do it yourself.