There's a nice article here from PJD about my experimentation : http://sandsoftwaresound.net/yamaha-pss-a50-stereo-mod/
It is very pedagogic with nice illustrations. I also think the differential approach is relevant in order to lower the noise.

I retro-engineered the Op Amp block, and... I get the same schematics.
Not so a suprise because the global picture is nearly the same as the one we can see in the PSR-F50 service manual.
I confirm also the resistors values and the 1µF capacitors, but I could not get a solid value for other small caps; that's why the schematics provided by PJD is interesting, because even if I try to reconstruct using the circuit simulation https://www.circuitlab.com/; it's better to have the good picture.

Retro_Schematics_PSS_A50_Small
The only major difference I have on my side is R501 and R502 for which I have 1,5k (instead of 2,2k, which is not dramatic since the divide bridge is still 1/2 in order to add the half direct voltage).

My initial idea with this mod was a kind of proof of concept, in order to identify if it's worth going deeper (i.e check if the stereo samples were present and played back). Which is the case.
Now, my target is to create a small PCB with the missing op amp, and replicate the mono circuit. Because of the components shortage, and since I plan to make my PCB via www.jlcpcb.com, I identified three possible alternatives : TL972, TS462 or MAX4477. For availability and cost reasons, I'll use the TL972.

Now the question is : only replicate a "Right" channel or replicate both in order to have a common design on the same PCB ? Well, I'll replicate only one channel. In case performances is worse (ground loops, noise... it might be easier to fix), and in case performances is better, I will just have to use two PCB (probably stacked, so I have to think about the design) : one for the new right channel and on for the left channel that will go into the headphones amplifier.
The left channel path for the loudspeaker will remain, using the original NJM2740M Op Amp

Most probably, I will unsolder the 4 caps C601 to C604 and wire the outputs of my PCB. TBC.

Now the Block Diagram with the integration of the Right channel looks like this :
BlockDiagram_PSS_Mono_Right