If you have an Amstrad CPC 464 and need a cheap ROM emulator then this uses just a Raspberry Pi Pico and a few components.
You can store multiple upper and lower ROMs on the Pico’s on-board flash storage. 1 lower and 8 upper ROMS are supported and a control ROM is included which allows the selection of ROMs to be loaded. The Pi Pico is overclocked to 225Mhz with one of the cores providing the ROM emulation and the other handling the ROM switching and processing commands from the CPC.
This is for the Amstrad CPC 464, so I am not sure if it is compatible with other models such as the Amstrad CPC 6128.
You can read more about the CPC Pico ROM Emulator on the developers Hackaday project page here.