I believe the last update (other than adding UNIXv4) to the official repo was a year ago, so it is unlikely it was ever tested with Trixie.
My fork is tested with recent Bookworm versions, and (extremely lightly) with Trixie. Regardless, you should always select "Compile" when using the kit and installer from my fork.
X11 is the "classic" display / window manager. It was replaced by Wayland some Pi OS versions ago, although you can still switch to X11 once your Pi has booted. One "gotcha" - if you are using the VNC remote console, after you switch you will be greeted by a blank grey screen that is unresponsive. You need to turn VNC off and then back on for the Pi to select the correct VNC server. And stuff on your desktop isn't carried over, so you'll need to get those shortcuts back. For the PiDP-11 ones, re-running install and selecting "Skip" for compiling and "No" to all of the operating system installations should get you set up.