I started the project by carefully gutting the viewers and bagging the parts based on their serial numbers for future reference. I then did some soldering and fitting of a 2.8" PiTFT screen, Pi Zero, four port USB hub, USB mini speaker, USB game controller, power supply using the vintage power switch on the back, and finally, a 32GB RetroPie game collection with a Star Wars theme including soundtracks playing in the backfround while searching through the game menu (courtesy of CoinJunkie and ArcadePunks).
These viewers look big enough for lots of internal parts, but that is deceiving. One third of the space is required for focal distance to magnify the image through the glass magnifier. Most of them are built out of die cast metal, so they feel nice to the touch.
Obi-Wan Kenobi would have said - "Your fathers's RetroPie. This is the gaming system of a Jedi Knight. Not as clumsy or random as a plastic case. An elegant system, for a more civilized age."
Great idea, I just finished mine!
Some hints for the software side: For NeoGeo and modern Arcade titles (like CPS1/CPS2), only PiFBA is fast enough on the Pi Zero. With the USB speaker, it will crash with a segfault because the USB speaker can only do 48.000 Hz and PiFBA does not support it - but it can easily be added in the source code and then it works fine.
With the display (MZDPI-VGA) and the standard configuration, PiFBA was running at 90 fps (way to fast). I fixed it with the following display configuration in config.txt:
hdmi_timings=480 0 100 16 59 640 0 20 126 15 0 0 0 60 0 31500000 1