Converting old organ keyboard to MIDI
If easy is what you're after then I would seriously recommend the Arduino UNO and an ICSP programmer.  The Atmel 8U2 chip that powers the USB can be re-flashed with a firmware that emulates a MIDI device (available here).

The number of IO lines can be expanded using any number of methods including simple shift registers, or SPI solutions like the Microchip 23S17 16-port IO chip, or things like 4-to-16 decoders (ideal for "scanning" matrix lines).