Full hardware mock-up is now operational (yes, it is damn ugly).

The current mock-up version is based upon:

(1) Arduino MEGA 2560

(2) Arduino MEGA SRAM shield (my PCB is modified for 56k RAM instead of 32 in the original version) https://hackaday.io/project/21561-arduino-mega-2560-32kb-ram-shield

(3) Arduino SD card reader; this version is for Uno, but runs on Mega with pins 10-13 using Soft SPI (library mods by David Mellis). Note that the standard SD library will not work if the classic Uno shield is transplanted to MEGA:

https://github.com/ptravnik/Retro_CALC/tree/master/SD2

(4) Piezo speaker on line 9

(5) Hardware keyboard on lines A0-A4. The driver is here (check the Morse code example!):

https://github.com/ptravnik/Retro_CALC/tree/master/HWKbd

(6) LCD screen 128x64 pixels on SPI. The driver is U8g2 from Oli Kraus (it writes Cyrillic and other character sets):

https://github.com/ptravnik/Retro_CALC/tree/master/U8g2

The keyboard PCBs have been ordered from https://www.elecrow.com/ Apparently they have been already manufactured and are in the mail. Hopefully I will have something less ugly in a couple of weeks. The order files are here:
https://github.com/ptravnik/Retro_CALC/tree/master/HWKbd/KiCAD

All hardware has been tested and fully operational. Now the next step: LCD terminal and RPN calculator part.