[jjjfdjunnmko]

Your best bet is probably the old MVS that was open sourced back in the 70s or 80s and lives on via an emulator called Hercules. Its sufficiently different to be worth a look, it's got a living direct descendent (although that has become increasingly turned into a Unix over the years), and there is enough of a community around it due to it having a very expensive direct descendent that the docs and other tools necessary for working with it are relatively high quality and up to date in comparison to most long dead operating systems.

The issue with it is that there is no way around having to do some pretty serious assembly language work for an instruction set that no longer exists and whose modern descendents probably microcode to hell in order to run some of the old instructions.