If the rubber ring on your Cameo 1 roller bar has broken, unfortunately, it cannot be replaced.
Since the bar where this rubber piece is fastened around cannot be removed, a new rubber ring cannot be placed back on. This part is intended to help ensure materials are held in place properly during the cutting process. The rubber piece backs up the white rollers that hold the sides of your mat or other material. It is not entirely necessary for proper operation. It simply attempts to hold down the material more firmly during the cutting process to prevent potential issues with the material slipping.
You can continue to safely use your Cameo 1 model.