I would check with Sew-Classics. I just ordered a 13 7/8" belt from them for one of my old Morse machines. They sell by size and not by brand. They are very helpful when you call, as well. I'm sure they can tell you what kind you need. (just a happy customer and not affiliated with them).

You will love this little beauty once you get her cleaned up and set up with new belts. I have a 21 and a 21A (my absolute favorite sewing machine). I too joined the Facebook group and the Yahoo group. I was able to get belts from one of the shops recommended in the Yahoo group. The price was very reasonable and if memory serves me well I recall that the gentleman was in Pennsylvania. I do have a service manual in PDF format that I can send you if you PM me with your email address. It will save you hours of frustration and is very detailed. I believe I also have a PDF of the manual if you need that as well. If you have any questions please don't hesitate to ask. I have learned a lot through trial and error and have pushed my two machines through many projects, piecing, free-motion quilting, baby apparel, heavy-duty repairs, etc. There is nothing that has stopped it in its tracks. Enjoy!!!