We had a similar dilemma. Didn't want to lose the original leaded lights, but the frames were completely rotten, so secondary glazing not an option.

Asked exactly the same question as you, but found no one willing or able to do that. Also, replacement in timber not only v. expensive, but also won't last as long as the originals did - apparently they don't season the timber like the used to do 100 years ago.

Eventually, reluctantly, went for uPVC replacements from a (family-run) company called Crestshield, who did an excellent job replicating the look, at least, of the leaded lights.

Well pleased with the results and the house is just so much drier, with no condensation (and accompanying black mould, if you don't stay on top of it) on the inside of the windows like we used to have