ADB shell: change IME method gives error "Unknown ID: null"
It sounds like your suspicion is correct, and the manufacturer has removed this functionality from the firmware. I'm afraid it's not as simple as copying and pasting some files: you'd need to obtain the source code of the firmware, modify it, and recompile to add the functionality, and likely you'd need to overcome whatever problems made the vendor disable it in the first place (such as that keyboards aren't made for such small screens).