Teensyduino project for using an OEM NES controller as a USB gamepad.
Internally NES controllers are essentially just a shift register(typically a 4021 or 74LS165) which presents the data from each button over a serial protocol(latch, data, and clock). So all we need to do is grab whatever the shift register is outputting and use it to prepare a USB packet.
If you just want an up and running USB NES controller, you can break out each pin from the male connector:
all you need to do is hook up CLK, LATCH, and DATA to a teensy & you're ready to go. You can also salvage a female receptacle from an NES or source one elsewhere.
Alternatively, for a more permanent solution, you can desolder the cable entirely & run the wires to a Teensy mounted internally.
Once you've chosen one of these methods, all you need to do is upload the code to a Teensy set to Joystick mode. The device should show up as either "NES Controller" or the vendor ID associated with your Teensy.
You can easily test the gamepad with either joy.cpl(Windows) or https://apps.nektro.net/gamepad/ although it's not guaranteed web-browser based testers will yield accurate results due to differences in how each browser implements gamepads.
This can also be used as a library for handling NES controllers although I haven't tested it with PAL controllers.