Hey, tracker has basically nothing to do with the stuff X11 would do, it’s “only” a client application, even if it is a big one, you can stop it with launch_roster and all your applications will still work basically.
The equivalent we have is app_server, the protocol is somewhat internal to haiku though and it’s not ment to be used directly by third party applications without using the application kit like X11 is.
We have some frameworks ported that many applications on linux use, or that some applications have as optional backends, those can be used for porting.
There is no X11 server on haiku as far as i know. (Unless you count booting linux in qemu)