If you want to build and use a Harlequin 128 kit, but can’t find a ZX Spectrum 128K ROM (such as the Toast Rack one), you can just use a ZX Spectrum 48K ROM instead.
This will run any game without any problem, as shown in this video: https://www.youtube.com/watch?v=3g-s2ZOStOs.
48K ROMs are way more easy to obtain (though we sometimes offer a couple of original Toast Rack ROMs in the webshop).
Underneath some photos of the small modification needed for using a 48K ROM in a Harlequin 128 board.
So you will need to solder a wire from pin 14 to pin 27, and bend pin 27 outside so it won’t go into the ROM socket on the Harlequin board: