>>1104992I don't see why the Sora wouldn't be able to handle those rings, I'm pretty sure they make a Sora crankset with a 50T big ring. And those 34.9/31.8/28.6mm numbers refer to the size of the clamp that attaches your derailer to the frame. If you have access to calipers, go ahead and measure your seat tube. But if you don't, you can just reuse the clamp for the derailer you already have on there.
However, I'm not so sure that replacing the FD is really necessary. I'm not an expert on compatibility, though.
You could also swap your bars for bullhorns, get some of those reverse brake levers (like on a TT bike) that clamp to the outside of the bar, and install your barend shifters in the ends like in pic related.
