I don't see a reason not to update LuaJIT still. Changes in Lua aren't just version numbers, it should be improving something, meaning that would be missing in LuaJIT.
As for forking, you can try, but I would warn you that one does not simply fork LuaJIT. Required is deep expertise in tracing JIT compilers, in assembly and in many different computer architectures. Nobody was really up to the task when Mike Pall announced that he was searching for a maintainer, before his eventual return.