Thank-you everyone, for your thoughtful replies.
I just thought of something! My camera has 116 spools in it; in fact it actually has 116 film in it! So, (and please forgive my total lack of knowledge on this subject) is it possible to simply use the existing backing paper from this old 116 film, and re-spool some 120 onto the 116 spool? If this is possible, then the frame numbers should still align perfectly with the window, and everything would (should?) work just fine, correct?
Is there a knack to re-spooling film? I will have to Google this to see what I can learn; maybe can find a detailed tutorial/Youtube video or something. As I stated earlier, the camera is immaculate, and it's indeed a shame that it isn't still being used. And I'm thinking that as I already have the 116 spools, and even 116 backing paper, then re-spooling is the best way to go, rather than attempting to modify 120 spools.
So, calling all experienced re-spoolers... do I need a degree in "Re-spooling 101" to tackle this, or am a worrying needlessly?