Ok, time for some schematics. First, a block diagram:
MIDI input module
The output from the opto coupler goes to RX on the Arduino, optionally via a switch or jumper so you can temporarily break the connection if you want to re-program the Ardino without removing it from the socket.
LED matrix display driver
You need two of these, with the second driver’s DIN connected to the DOUT of the first one.
The easiest way to build this is to source a few LED matrix kits off ebay or similar (just search for “MAX7219”). They ship with a 32 mm 8×8 LED matrix, which can be used as is for the small grid. For the larger 8×6 display, you have to roll your own LED matrix, but you can wire it up to match the 8x8 matrix pinout and use the kit’s PCB as is. Or if you make sure to get a kit with a 24-pin DIP (standard though-hole IC) instead of a SOIC (surface mount), you can just keep the chip and build the circuit on perf board.
Code to follow.