Machine screws have size and thread count. There are gauges that can be bought and kept around the shop to check. It's size and thread count per inch. So you could have a 6 - 24, Size six, 24 threads per inch. If you can get it apart and take it to a good hardware store, they should have a gauge or just pull out a few screws and fit it. Once the size and threads are found you can pick any nut, knob, screw with that size. Works for any machine screw, knob, nut on anything.
If it's missing or shot use what I have or fix it up from the other posts. If you look up screw gauge on amazon you can keep on in the shop. Sometimes they are in hardware stores, but not recognized or sold much. RJF
NOTE; Screws, nuts come in imperial and metric like other things. Gauges are also combination of metric and imperial either separate, in sets or combination. RJF