I'd say that the proper vintage replacement for that derailleur would be a Suntour VGT. At least that's what we used to put on when the Simplex derailleur body wore out. There has to be a few of those still about because they produced and sold them for a very long time and they were built to survive a nuclear attack.

If you just want to make the bike rideable, I'd go ahead and slap on the Tourney. It comes stock with it's own claw attached.