I have successfully modified one of my EPs following Zozo's hints.
This is the tasks list:
-Installed a "programmable in situ" 128K AMD FlashRom with a switch to select between the two 64K banks. The last version of EXOS has been programmed on the two banks.
-Un-welded the original Z80A clone and installed a Z84C0020 on a socket.
-Followed the overclocking procedure in this same thread, that implies:
-Separated the processor clock signal from the system clock frequency.
-R12 short-circuited
-C7 opened
-Added a pull-up resistor array on the Z80 data bus.
-Installed a 20Mhz clock crystal to drive the processor.
-Installed a 74LS157 to select between 8Mhz and 20Mhz(halved frequency on the Z80) while the computer is running.
-Put a switch to make the selection.
-One of the commutations of the switch has been also used to select the colour of the main power Led.
This EP had already a 1MB Saint Ram expansion installed, and the task remaining to me is to remove the two 7805 regulators and install instead two 2A Traco-power that I am still waiting.
Once closed I hope not have to re-open this computer ever again...