Can't you just put a 620 roll into a 120 camera? The only thing that needs adjusting in the spring tension on the roll, right?

620 rolls were narrower than 120. That is why 120 film would not fit in a 620 camera, the compartment was made smaller. All the other dimensions were the same as 120 rolls.

But you are going the other way. Your 620 rolls should fit right into any 120 camera. You might need to reach in and bend up the spring to add a little tension.