UPDATE: I set up a blog where I'll be posting how-to guides for this project. You can also enter there for a chance to win the one I'll be building! http://www.sudomod.com
I made a RetroPie handheld using a Raspberry Pi Zero and an original DMG-01 Game Boy. I modified an original Game Boy cartridge to act as an SD card adaptor for the Pi, so I can have the same satisfaction of putting a cartridge in and powering it on like you would on a regular Game Boy. :)
Feel free to ask any questions in the comments and I'll try to answer.
Here is a list of parts used:
3.5” composite display: https://www.adafruit.com/products/913 A word of advice on these composite screens: make the wire connecting it to your Pi as short as possible to reduce noise. Adding some shielding helps too. You can get them cheaper elsewhere, especially if you rip one out of a backup camera system on ebay or something, but the Adafruit one is well documented, easy to convert to run on 5v, and I knew the size of the controller board and screen housing before buying, so that was nice.
Teensy LC: http://www.amazon.com/PJRC-Teensy-LC/... I used this to wire up the controls and have them show up as a USB keyboard in Emulationstation. You could use GPIO for the buttons if you want, but I wanted a Teensy to add some multicolor status LEDs later on down the road anyway. Plus it makes it easier to upgrade the Pi Zero in the future if they come out with a Pi Zero 2 or something.