Adding MIDI output to a Yamaha PSR-78 keyboard - is the Arduino fast enough?

I have a Yamaha PSR-78 and wish to do something similar to what was done here: Code Tinker Hack: How to turn Piano toy into MIDI keyboard (using Arduino/Atmega)

The difference being that I would like to add MIDI output rather than replace the whole CPU as was done there.

From what I can gather (and I may be wrong), the main chip (YMW282-F) scans the 49-key keyboard matrix with an approximately 500Hz square wave* that pulls down in sequence, each of the 9 I/O lines that make up Port A0-A7 and B0
(* I have scoped one of the lines with a CRO and got about 2 Div at 1mS/Div setting, I believe that calculates to 500Hz)

The 6 lines from Port B1-B6 are connected to a pull-up resistor array, and receive the returned data from the matrix when a key is pressed which pulls that input low, and obviously the CPU decodes the result (in addition to whichever of the 9 lines was set low at the time) and plays the appropriate note(s).

I am hoping to connect an Arduino-programmed ATMega to monitor all 15 lines and do some decoding based on the previously mentioned project, then output the result via a MIDI connection.

My question is - is it feasible? Can an ATMega read 15 lines and output the appropriate MIDI data fast enough? Do I need a higher clocked microcontroller? Or should I be using a CPLD\FPGA instead?

I have attached the only schematic of PSR-78 I could find (A photo of a service manual I found on eBay) and also a pinout of the YMW-282F in case anyone needs clarification on how everything is connected.