Grenoble has some good bike shops for tandems. From what I recall, Routens stocks the longer cables.

That is a nice-looking bike, I would try and keep it in original form.
It would be a shape to split up the transmission and put a modern MTB crank on. Are the cranks Shimano or Specialities-TA? You may be able to fit a smaller chainring on, but not by much. Far better to start on the rear cogs.
You can get rear cogsets up to 34 teeth. You may need a modern MTB derailleur to cope with the capacity