A few files I coded to connect an Arduino Mega 2560 to a keyboard with a broken MIDI controller.
Keyboards are wired in a multiplexing matrix. In order to figure out that matrix, all wires are connected to any of the Arduino pins. keyboard_init.ino tests if there is a short circuit (key press) between any pin pair and sends them via serial.
When the matrix is known, keyboard_midi.ino can be modified accordingly, provided the pins of the rows/columns in order. This Arduino Sketch uses the Control Surface library to send appropriate midi commands via USB.
The Arduino Mega 2560 (likewise the Ardunio Uno) needs to be flashed with a custom firmware to appear natively as a midi device to a computer. For this I used USBMidiKliK. It makes it possible to upload new sketches to the Arduino by shorting the MOSI and GND pins when connecting it to the computer. Otherwise, it will appear as a midi device. Flashing the firmware can be done with dfu-programmer with the commands in script.sh. Serial does not appear to work when using this firmware. The original firmware can be found here.