If you cannot find a source of the spring belts used for Meccano, the garter springs from lip seals (oil seals ) are the same. To shorten, you need to cut the spring and then carefully unwind the wire of the cut end, to expose the tapered end. Having arrived at the required length, you then twist to the free end, a few turns, in the opposite direction that is required to "screw" the belt back together, so that when rejoined it is has minimal twist.
Howard