I would grab Rainer's patch for the JX-8P from the files area.
Load the *.BC2 into a text editor like Notepad and you will see his notes and you can have a look at the BC code.
Grab Mark's BC Manger and load it into that and you should have a good place to start.
The sysex lines will have to be changed along with the max and min values.
You might want to rearrange the layout as well to suit you needs.
Free free to pop here and ask anything you want.
All the best
Royce