As far as I know, all road handlebars have always been made to the same diameter in the area where the brake levers mount. The 26.4mm diameter of your Cinelli bars refers only to the center section where the stem clamps on to the handlebars. All handlebars taper down to a smaller diameter in the area outside of the stem clamp. If you have not yet tried fitting those brake levers to your bars, give it a try before you waste time looking for a solution that likely doesn't exist (because it doesn't need to).