This is a schematic and a PCB for a single board MegaDrum 56 inputs with 4851 MUXers.
All KiCad and PDF files in one zip file:
Warning!
To the best of my knowledge the schematic and the PCB are correct but it has not been tested by me.
1. EEPROM chip (24C256), PIC18F2550 and 6N138 are optional depending on your requirements.
2. Schematics for LCD and controls are the same as in viewtopic.php?f=3&t=266
3. Pinout of the pads connectors is the same as in MegaDrum Kits in viewtopic.php?f=3&t=1420