You’re sure? The center part of the handlebar bulges to 25.8 or 26 or 31.8 mm and is clamped to the stem, but the rest of the bar should be 23.8 mm, which is the usual road lever size. (22.2 mm is the standard MTB bar size.) On the two lower pictures with the Thomson stem, you can clearly see where the bar is sleeved.
You should be able to find an old set of DiaCompe brake levers on eBay with clamps that will fit a standard bullhorn. The clamp is just held to the lever with one nut inside the lever. Swap the clamp and nut and you’ll be good to go. If you can’t find something, I might have a set in the shop that I can cannibalize.