"So when boat shopping, being leery of a Perkins with a 30+ year old manicooler is wise." There are plenty of Perkins engines out there that have a better heat exchanger design, I suggest the OP focus on those."
The above is the position we took when shopping for our boat. Our Perkins
T6.354 has the heat exchanger and tank on the port side, not the manicooler. . The heat exchanger end caps are easily removed to inspect and clean if needed. The Perkins smokes VERY little at start up and goes away within a couple minutes. Our Perkins has been a reliable economical power choice with no complaints for 6 years.