So here is current situation: the 5.9v zener reference has been replaced by a 2DW233. First quick check was a bit dodgy: this ref is normally 6.2v, but actually consists In 2 zener head to head, so in practice a 5.5-ish v zener in series with a forward one (so .6-ish v). So I've used only one of the zener of the 2DW233 in series with a germanium diode. I also used a random opamp I had but had not the correct pinout (it's a dual opamp). The result is a bit dodgy as I said, but it worked: 5.87v
The ref voltage is a bit off (6.0v) but well within the required range (+/-0.3v). After a quick adjustment, it's starting to look better
But it was pretty unstable, mainly because the 2DW233 was drived too low with current schematic: at 1.5 mA instead of the expected 7.5mA (for the 233). So I played a bit with ltspice to find better values for R61 and R63, and I also replaced the opamp with a 741 I found while cleaning a bit my bench :-)