OK, I’m going to risk sounding dumb now, but is it really worth the effort to replace the manifolds? Maybe ensure a few isolators are put in so that just the cold or just the hot can be shut off, but given that the horse has bolted will enough time be saved in the future to repay the extra time and delay (and the knock on effects of a pissed off tradesman)?
If there’s a leak you shut down, deal with it pronto or whack a stop end on the offending bit till you get the parts.
I maybe guilty of Hep2O think here, but I put isolators everywhere in our last house and in 34 years most of them never got closed off.