Does it matter if the feed spool is 120 as long as the take-up is 620?
Frame spacing comes from winding on, which comes from turning the takeup spool, so the film travels the correct distance from the right number of turns, which depends on the diameter of the spindle of the takeup spool, ergo you must have a 620 spool in the takeup position, obviously.
The feed spool just turns freely depending on how much film is pulled from it.
So if you can get a 120 spool in the feed position and it still fits and turns, and feed to a 620 spool in the takeup position, would this not also work?