I'm about to buy another old bike. It comes with Suntour Cyclone derailleurs and shifters mounted on the stem.

If the condition of the derailleurs are any good, the plan is to replace the Simplex with them. Hopefully the Suntour group is compatible with the Simplex cassette.

That's my plan by now. Let me know if it's common practice to update - makes the search function so much more useful I think...