I’m certainly not an expert, but for all RGB mods for the N64 I would recommend the SFC SCART cable, as the PAL cable is wired differently, and if you use it, you could fry your equipment.
With regards to that mod, I’ve never seen that particular circuit board before, but it’s a standard THS7314 mod it looks like, and not a very good one. If you look at the board, he has terminated the outputs with 75 Ohms, which is correct, but not necessary if your SCART cable already has 75 Ohm resistors in it. I’ve also noticed he’s jumped the capacitor terminals on two of the colour outputs, whilst leaving the third colour with its capacitor in place. These capacitors are all components usually present in your SCART cable, the only way to know for sure is to open it and have a look. Also, the eBay listing says his consoles are wired for CSYNC, but there is nowhere for the CSYNC to run on his circuit board. The likelihood is he is using older Japanese models that have TTL sync already hooked up to the multi out, but without proper attenuation via a 470 Ohm resistor in the SCART cable, you again risk damaging your equipment. My advice? DO NOT plug that wonky N64 into your OSSC via any means, the shoddy workmanship may damage either your N64, your OSSC, or both.