There really isn't a way to boot into RetroPie on there, but if you use Retroarch as your emulator, it should minimize the amount of re-configuration you need to do.
RuanCaiman
Pi Zero is $10. You'd likely need to pick up a micro-hdmi to full size adapter. If you're already invested in USB controllers, then you'll need an OTG adapter + USB hub. Otherwise, it should pick up bluetooth devices.
There really isn't a way to boot into RetroPie on there, but if you use Retroarch as your emulator, it should minimize the amount of re-configuration you need to do.