I know it has been done before, but I too have written an assembler for the KENBAK-1 and its replicas. Actually, I wrote a configurable assembler for a TTL CPU I am building, but to demonstrate its configurability, I created a configuration for the KENBAK-1. The syntax isn't the same as the original KENBAK-1 assembler, but all instruction functionality is present, and it can even generate language extensions for VS Code and Sublime Text so you can get syntax highlighting for KENBAK-1 assembly.
If you are interested, you can find out more here:
You need to have python 3.9 or later, and preferably you have a python virtual environment set up to install this compiler.
I'd love to get any feedback you might have. Note that the assembler is still a work in progress and still a little rough around the edges when errors occur.