GoKawiilFinally it works on the Raspberry Pi 3b! Here is an image of the running system; I'm using the original Raspberry Pi monitor and a Lenovo ThinkPad TrackPoint keyboard which has three mouse buttons (as suggested for the Oberon system).
I attached a ready image (oberon-rpi3.img) which you can flash onto an SD card if you want to run the system on your Raspi 3b yourself. To do so, unpack the image; on Linux, run this command to flash the SD card: sudo dd if=oberon-rpi3.img of=/dev/sdX bs=4M conv=fsync status=progress && sync ; on Windows or Mac you can use the Raspberry Pi Imager or e.g. Etcher.
Of course you can also build the whole system yourself, and even the toolchain if you want. I attached the required Raspberry Pi bootfiles and a pre-compiled toolchain for Linux x64 for convenience. The Linux build script is in the arm32/build subdirectory, also a script to flash the SD card.
The image should also work on a Raspi 2b (>= v1.2) or Zero 2.
Why those models? They share essentially the same hardware architecture and can still be regularly ordered. The Raspberry Pi 3b will remain in production until at least 2028 according the the manufacturer. The Pi Zero 2 will even be in production until at least 2030. Even the Pi 2b v1.3 can still be ordered. Looking at the Model 1 and 2, it is reasonable to assume that lifespans of 3b and Zero 2 will be extended. Also migrating the present implementation to Raspi 4 looks feasible so far.