This work has been done by Paul Hackmann.
The CD-i Gamepad Adapter is a cable designed to connect an analog PC joystick/gamepad to a CD-i player. The cable handles the necessary calibration of the PC joystick/gamepad, and converts the data a format understood by the CD-i player. With the CD-i Gamepad Adapter, you can enjoy CD-i gaming with the controller of your choice.
Paul Hackmann got requests to release the design for public use.
He decided to open some of the documents to the public.
Those files should be enough for someone to recreate a working adapter (not necessarily exactly with the same look), assuming people have the ability to make circuit boards and compile some software.
It lists the PIC16C710 on the schematic, but the PIC16C715 datasheet was used.
They are pin compatible.