Your machine has the same layout of my mother's (I inherited the machine). Mother's was a Singer. Belting wore out or became old and fragile. Users re-belted their machines using sewing machine belt stock. Belting was a leather stip with about 1/4 to 3/8 square cross section. The belt was cut to length and the ends fastened together with a metal clip. Sizing the belt: The ends of the belt will be butted together and fastened. Plan the lenght of the belt accordingly. To find the correct length, put the machine into its normal work position and thead the belt stock over the drive and driven sheaves ending on top of the upper sheave (head wheel). The belt has some stretch so it should be a bit short when relaxed. Mark the belt and cut a bit short, maybe 1/3 to 1/2 inch short. Stretch the belt to get and idea of ahow much to shorten. If this is your first time at belting, don't cut as as short as your first estimate. If the finished belt it too long ( and slips), it can be shortened; it can not be lenghtened without whacking it in two and adding a piece. Fasteneing the belt: The butt ends of the belt will be fastened with a metal clip that is shaped like a hog nose ring (a really small one). The belt ends are butted together and the clip pierced through the belt ends. The sharp ends of the clip are then bent flush to finish the job and to secure the clip. If you can not locate a suitable clip, the belt ends can be fastened another way. Drill a small hole through each end of the belt and lace together using small copper or brass wire. Pass two or more loops of wire through the holes, clip the ends to length and secure the wire ends by pushing these under the loops. You could also twist the ends together, trim, and bend the joint flush. You'll want the rough joint to be on the outside surface of the finsihed belt. Since square section belts can twist, there is no guarantee the rough part of the joint will stay tipside, therefore, the smoother, tucked ends are recommended. If you are trying to restore to authentic condition, use the metal clip, otherwise use any method that works for you. Look at other old machines to see how the belt end is fastened. Belt stock: In today's world, small section leather Belt stock may not be available. Look in leather craft stores and harness makers. Available belting stock may be a leather look alike or some other material. For authentic look, use leather or a leather look alike. Other stuff can be substituted including braided cord - it won't have the proper appearance. Whatever is used, the finished belt should have a little stretch to maintain tension and to facilitate thread and unthreading the belt. Look as the shield surrounding the driver wheel. There should be a spring loaded lever for putting the belt on the wheel and taking it off. When the machine is not in use, it should be unbelted to relieve tension in the belt and to prevent accidental movement of the sewing needle. Also, the machine needs to be unbelted for raising or lowering the machine head into its storage position under the top.

Can you post a pic somewhere? It doesn't sound like a machine made by National in the USA, more like a post WWII Japanese machine with a National badge. No American made Nationals had zig-zag or buttonhole capability.

Machines that fetch $20.000 are new industrial machines. The very rare collector's item in pristine condition have dollar value, but there are just too many old sewing machines about and way to few buyers for the value to be very high. In the world of sewing machines, only those made prior to 1900 are "old". On the other hand, these old machines have their advantage over newer ones. They are solidly built and with a bit of time and attention you get them to sew again, and they still do a very good job. The quality is often outstanding compared to new low priced machines, and even if they can't do zigzag or anything other than straight stitches they make a perferct stitch and handle most fabrics with out fuzz. That's why they are still valued as working machines. History and age add to their charm. I can't help much with dating, it would take a bit more research. Does it have reverse stitching? When you get closer to 1940 most models seems to be able to do reverse stitching, but not all. In the 50s built in zigzag became more common, but it has existed at least since the 1930s on a few domestic models like Pfaff 130. For comparison Singer made a updated version of model 99 (straight stitching only) until the late 60s.

I can't help you, but there is a forum on quiltingboard.com for enthusiasts of vintage machines. I'll bet someone there can tell you a lot about it. They seem to be very knowledgeable.