The schematic is available in 'MegaDrum kit - instruction manual.pdf' in
viewtopic.php?f=3&t=1420 but it is of no use for pure DIY with STM32 as STM32 with protected bootloader comes only with kits or modules.
For Atmega based 56 inputs schematic see
viewtopic.php?f=3&t=1623 - but with latest the firmware version it supports only 48 inputs due RAM limits.