the ZX Touch Manual explains how it's done under section 3.2.7
"To ensure full compatibility with games, it is recommended to use the original ROMs from the ZX Spectrum 48K and ZX Spectrum 128K computers. These ROM files should be placed in the "SD:\ROMs" directory on the SD card. The file names for the ROMs are predefined as follows: "48.rom" for a 48K computer, and "128-0.rom" and "128-1.rom" for a 128K computer."