Very nice looking H3k. I have a Director Pica on my repair pile. I like all the neat Hermes typefaces. I think that is why I have several H3k machines. I also prefer the green ones over the ones with the white keys. The green looks much nicer. I have both,, but the green have a distinctiveness to set them apart.I find the round body ones are nice to use than the square body, but I like the design of the square ones a bit better.Good luck with your knob project.
Talking of directors, Fifty and Raining could be a movie title.
Hopefully you'll find a parts machine with the platen knobs you need. That'd be the best option. :)