The Perkins 6.354M is a great engine. And, the parts are still available. I have two in my boat.
The manifold it is in current production from Mesa Marine in Alabama (
Marine Exhaust Systems of Alabama, Inc.). It seems to cost $2500-$2800. But, unlike the original it is stainless vs carbon steel, so it should last a lifetime.
If one wants to replace the heat exchanger and manifold with a combination manicooler, they are in current production from EJ Bowman in the UK (
https://www.ej-bowman.com/)
Either option is available directly from the suppliers and also through resellers, such as TAD Diesel in VA (
TAD for perkins engines, perkins diesel, perkins marine, perkins parts, perkins generators, perkins service) and S&W Diesel in CA (
https://www.swdieselinc.com/), among others.
The cost for the manicooler is about $2,000 but there is an install kit that is needed to convert from a separate manifold and heat exchanger to the single manicooler. It consists of hoses and other small parts. TAD Diesel apparently wants another $2,000 for the install kit. I think there was generally a sense on the forum that it'd be worth getting a quote from another supplier, e.g. S&W Diesel, and/or ordering straight from EJ Bowman, or putting together the kit from individual parts one's self, before spending that much on the install kit from TAD Diesel.
In any case, the parts for the engines are still available, including rebuild kits, coolers, manifolds, etc. The only part I know of that i hard to find is the contra-rotating raw water pump. It is rebuildable using the same kit as the regular raw water pump. But, if the housing breaks, one would need to repair it, cast a new one, or improvise with a 3rd party pump. I haven't seen those for sale for a while.