Engine components are available from multiple sources for all Perkins engines.
But with many older diesels, the rub comes when you need to replace marinization parts, like exhaust manifolds and risers. Since the marine parts weren't used in farm and industrial applications, they may only be available from after market suppliers.
Like with the Perkins 6.354 the water cooled exhaust components can be thousands of dollars for each piece. You might want to check on the availability of those components.
Marine Exhaust Systems of Alabama (MESA) was custom building the exhaust systems. They have an online catalogue. Marine Exhaust Systems of Alabama, Inc.