Why? Why not. This is how it was done:
Instructables user dany32412 pulled an Xzibit, he built a Nintendo NES system into an old school NES Cartridge.
Inspired by Kotomi who completed a similar project, he sourced a NES On A Chip (NOAC), a cloned consol which is essentially a fully functional Nintendo Entertainment System on a chip. It was then shoehorned into a NES game cartridge by way of dermal along with two original NES controller ports making it possible to retain the iconic controllers.
After a bit of soldering, a switch, a 72 pin connector and an LED bulb, the NES inside a NES Cartridge DIY project was complete.