If you are good at electronics and soldering, there's a relatively easy modification to the internal expansion module to upgrade it to 256 kB.
There you can find the description. Unfortunately, in Hungarian only.
The most important parts are the linked schematics (1. ábra and 2. ábra; pointing to the top and bottom parts of the very same picture). The top part of the schematic shows the modification on the address multiplexing. The bottom part shows the modification on the DRAM refresh logic.
The instructions go something like this:
Replace the 4164 DRAM ICs with 41256s.
A16, A17 and A18 must be connected to the (top) 74LS151 multiplexer. Its output must be connected to pin 2 of U110.
Disconnect A16 from pin 2 of U112 and connect pin 2 to ground.
Disconnect A17 from pin 1 of U110 and connect pin 1 to +5 Volts.
Disconnect A18 from pin 2 of U110 and connect pin 2 to pin 5 of 74LS151.
Switch SW will disable the expansion if closed and enable it if opened in the contiguous ECh-F7h segment range.
Disconnect pin 12 of U109 from pin 9 of all DRAM ICs.
Connect pin 1 of all (new) DRAM ICs and connect them to pin 12 of U109.
Connect pin 9 of all DRAM ICs to pin 5 of (bottom) 74LS151 multiplexer.
MUX signal can be taken from pin 1 of U108 or U109.
The three TTL ICs can be piggy-backed on components already there on the board but the DRAMs are recommended to be seated in sockets.
Unfortunately, I know next to nothing about electronics, therefore I can't guarantee that my translation makes any sense at all.