Give it up and build your own .....
Game are selling a bluetooth/USB zx spectrum retro keyboard for £29.99, open the case and put a raspberry pi zero w in it (theres room for a full size raspberry Pi 3 if you have a dremel and a glue gun). Either plug cables in and trail them through the back of the case or get fancy with adapters and make it look pretty with fixed sockets on the back.
The entire project can be done for less than £50.
Put Batocera Linux on the microSD card. https://batocera-linux.xorhub.com
Plenty to think about here: https://blog.hackster.io/a-beautiful-raspberry-pi-powered-zx-spectrum-489deb2020ab
Job done.