So you found an old medium format camera, and whilst it seems to work the currently available medium format 120 roll film won't fit because the spool is a little too fat and the drive teeth are too small to fit the 120 spool, It probably needs 620 film which if you can find it is stupidly expensive, there is at least one company making new spools, and several doing as we are going to do here selling 120 film wound onto old 620 spools and you're paying for the time.
Step 1: 120 V's 620
The film itself is the same stuff, Which is why we are able to do this. A paper light proof backing and a strip of film.
The differences are with the spools, 120 has a fatter core, wider thicker ends, and a larger drive hole for winding on the film
Step 2: Requirements
Access to a dark room, or changing bag you are going to need these to process the film later.
As that is covered here https://www.ilfordphoto.com I have no connection with the company other than using their film and chemicals which I buy locally.
2 empty 620 spools a new roll of 120 film.
For testing an old camera basic black and white is best, I used ilford FP4 plus. The optics on many old cameras don't give good results with colour film anyway, 120 roll film is available in standard c41 processing negative, and E6 slide versions too so once you've proved your old camera works you can try colour if you wish too.
Step 3: Check and Recondition
Most of the 620 spools I've encountered are steel, I've seen one with an aluminium core, they need to be clean smooth and free from rust, whilst they tend to be blackened, bright spots won't matter, rust like mine has will. This rust is thankfully light a good scrub with a new wire brush took care of most of it followed up with some fine emery cloth, 800 to 1000 grit wet and dry would do. I should have followed up with some rust killer and sprayed it lightly with matt black paint, I made do with a black ink marker
Step 4: Transfer the Film
This has to be done in total 100% darkness, the illustrative pictures are done with just an old backing paper and no film.
Place the empty 620 spool, a new roll of 120 film and a small rubber band in the changing bag, or on the work top in the dark room where you can find them. Put your arms in the changing bag/ close the door and turn off the light.
Locate the 120 film break the securing tape, feed the tab into the slot in the 620 spool, wind the backing paper and film from the120 spool to the 620 spool keeping the roll tight, secure with the rubber band
Step 5: Use or Re-roll
At this point you could load the film into the camera and use it. However there are a few disadvantages to this, the indexing will be back to front, you don't get an indication you have nearly reached the next frame, the roll says it is exposed when it is new and unexposed when it is used, which could get very confusing if you are preparing more than one roll. You could drop a second 620 spool in the bag and repeat the process giving you a correctly wound roll of film,or use the camera if its design permits as this one does, load the film close the back wind it to the used spool, without tripping the shutter. Remove the film and re-load it.
Step 6: Use Your 620 Film
You've only bothered doing this because you've found a camera you want to test. So try it out see if it still takes good pictures, does it leak light, if it's more complex than the one I found which has fixed focus, shutter speed and appature does it focus.