No mods required -- just unbolt your Simplex with its claw and bolt in the claw-mounted substitute of your choice. The Huret Svelto suggested by big chainring would be a superb choice and country-correct, but any of the suggested Japanese derailleurs would work well, and there is ample precedence for European bikes with factory-installed Japanese derailleurs, most of which will require a somewhat longer cable housing.

If the end of your shift cable is frayed, or if there are any broken strands anywhere along its length (particularly at the shift lever), just do what the pros do and replace the cable instead of trying to salvage it.