Y cant you use terminal keyboard on normal pc?
The controller on the board runs at a totally different speed. So its not a simple of changing the RJ9 for a 6pin mini din.
You'd need to map the board out and solder a teensy in there. Well worth the effort as they are well built boards.
Would be nice to see someone from the UK keep the board in tackt, and take it on as a project due to the condition its in.