Unless something dreadful has happened to it, there shouldn’t be any need to replace NACA5.
The plugs will become oxidised over time and yes, this will affect the sound. You will be delighted to hear that Naim’s recommended solution is quick, easy and free of charge: simply unplug and replug connections several times to clear any oxide. How often you do this is up to you, but about once every six months is probably a good compromise between an ideal world and reality.
Naim (and many here, though perhaps not everyone) don’t recommend cleaning fluids of any kind.
I’ve been running the same set of NACA5 since 1996 (replaced 2 weeks ago on a whim with Witch Hat Phantom), but I’ve had no problems with the NACA5, and weren’t it for a lockdown-induced need to spend money on the stereo would still be using it.
If the plugs are corroded, or the solder is cracked , I would get new plugs soldered on. If the exposed copper cable is corroded or damaged, I would cut it back and resolder, bearing in mind that corrosion may have crept under the insulation, so be sure to strip it back until you find good clean wire.
If there’s no further damage, I would just keep it.