In my old boat, I replaced the 6.454 MGT (185hp Turbo version, originally with the "breadbox" coolers) manifolds with new ones from www.mrcool.us
. I don't remember who made them, it might have been Mesa. But, they were readily available. That was, I think, in 2016 and they still look current online. They were quick ship items.
Just before I bought my new boat with 6.354(M) NA engines, the present owner replaced the header, manifold, and heat exchanger with a combined manicooler (Bowman brand). From what I can tell from the paperwork, this was at about the same time and was also a quick-ship item. They still look to be readily available online.
TAD Diesel in Virginia and S&W Diesel in Wilmington, CA are also able to supply parts for these engines pretty readily. Never had a problem getting anything I needed.
I noticed that my boat's prior owner also was able to get good used parts for some pretty odd items -- like he got "new" valve covers. I can only guess the other ones were probably bent up around the screw holes leading to some leaking. Those engines are a little bad about that because there is so much distance between the screws on the cover that they can leak if the gasket isn't perfect, and over-tightening to fix it adds another problem.
My boat's PO seems to have used a ton of gasket sealant instead of the actual gasket, so I'm assuming he had trouble with the new covers and gaskets, too. It'll have to clean it up and do it again if I ever need to do a tune up and check valve leaf. But, c'est la vie. It is nice and dry there now!