The red window is bogus anyway. Years ago, films did not have sensitivity to red light and these windows may have afforded a measure of protection from accidental exposure. Today, with the exception of a very few orthochromatic films, all B&W camera films are sensitive to red light, and the red window's usefulness is nil. The backing paper on 120 film is light proof and that should be enough to do the job. You could cover the hole with a piece of black tape, removing it to reveal the numbers when advancing the film, if you feel you must.

If you can find a couple of old 620 spools (they come up on ebay now and then) it is fairly easy to rewind 120 film onto the older 620 spools. Of course, this needs to be done in total darkness.

I'd relegate the 616 camera to the display shelf. Not worth the effort of modifying it for 120 film.