Basics are easy to replicate, but one of the reasons why org is so useful is because it is tied into the emacs ecosystem, so you can write extensions/configuration tweaks in lisp. You can hookup agenda (calendars), etc, etc, and those things don't really translate so well to external tools.
If you had to write a lisp interpreter, and fake "bare minimum" compatability? At that point you'd be better off just running emacs for real.
https://orgmode.org/tools.html
Basics are easy to replicate, but one of the reasons why org is so useful is because it is tied into the emacs ecosystem, so you can write extensions/configuration tweaks in lisp. You can hookup agenda (calendars), etc, etc, and those things don't really translate so well to external tools.
If you had to write a lisp interpreter, and fake "bare minimum" compatability? At that point you'd be better off just running emacs for real.