I use commonly O ring as belt for my engines, as I have a large stock of large dia and small section, the right tensioning is not always easy to set, too much stretched, and a lot of energy may be lost.
But for a period engine, this type of belt seems an anachronism; for a steam engine governor for instance a simple round leather lace is much nicer, but not as much as a flat and thin leather strap, IMO.