So I'll describe here the first aspects that have been discussed with @Sylvain JULIEN.

The intended system contains:

• a decimal keyboard (a phone keypad ?)
• a "computer"
• a multi-digit display (probably multiplexed LED)

The first and last are Arduino-level circuits that many should be able to handle, but Sylvain wants the "computer" to be an actual dedicated electronic circuit. This implies a FPGA, which in turn requires a VHDL description and a suitable FPGA board.

There is one FPGA board that would fit the bill (already available, small, cheap, sufficiently convenient, and enough hardware resources) : The SmartFusion2 Creative Development Board (I have the MicroSemi version dating from before it was bought by Microchip)

• M2S025 FPGA (27K LUT4 with carry chains, 31× 1K×18 SRAM blocks, several 18×18bit DSP blocks...)
• 64M Bytes DDR2 SDRAM
• integrated programmer
• Arduino-style header pins

The issue is to get the free license, since Libero might not work for this very reference.

I have one but Sylvain is not able to use it.

Synthesis licenses are a big problem and it pops up again with the other available/suitable board : the older Actel Fusion Advanced Development Kit. I have several that contain a venerable AFS1500 (datecoded 2008), 32b×1M StaticRAM, and many other awesome features, but require a Gold license and a FlashPro3 programming dongle... I have them but they are very niche devices and Sylvain might not be able to use them either.