Well done you seem to be having fun, and hopefully will be able to continue doing so.

Came across this site 64Studio which might be able to help with the original OS.

While the Linux OS was open(As is any Linux distro) the keyboard OS was not, as the manufacture would not have made any money if everyone could modify it.

The Lionstracs was not unique, as Wersi have been making computer based keyboards and organs since the late 90s, first with the OAS system (1999-2014) and now the OAX system (2014- On), however as it has always been Windows based, it allowed the designers much greater flexibility with hardware and software compared to Linux. (Both OAS & OAX can use VST instruments to expand the sounds etc.)
As I mentioned at the biggening, enjoy your project as I am sure you will get great satisfaction from it.

Bill