Bob wrote:

But there are some folks that say "upgrade" to more modern model.

Modern isn't always better. I don't know what is going on with small marine
diesels now but I fear my recent experience with aircraft maintenance may be
relevant to boats. If anyone has data to add on these points, I'd
appreciate hearing it.

In aviation, things that used to last for hundreds of hours a couple decades
ago began failing very rapidly about a decade ago. This applies to vital
engine parts like valve lifters and camshafts as well as flight instruments.

The reasons are all the globalization issues. A turn coordinator used to be
built in a place where someone who had been assembling them for 20 years
knew that if one part swung back and forth just a certain way when trial
assembled, it was good to continue. Otherwise the parts needed some
tweaking. The company gets bought by someone who thinks they can make a
profit by increasing effeciency so they fire everyone and hire younger
people who will work for less. All they know is that part A goes into part
B and they have to assemble 20% more than the old crew did or they'll be
looking for a job.

There is a specification and a procedure for heat treating valve lifter
faces. Somebody, or a crew who has been doing it for a long time, knows
something like the fact that letting the kiln cool down with the parts in it
instead of just taking them out is the difference between parts that fail
prematurely and those last. This never makes it into the specification
because it's just the way they've always done it. The company gets sold and
the production is moved to China where they follow the specifications to the
letter but the parts still start failing half way to overhaul time.

A lot of things like engines were designed to be built by artisans who cared
and understood them and have not made the transition to modern times
gracefully. A thouroughly modern engine built on a thouroughly modern
production line with the advantages of computer machining, etc. may be
better. Automotive engines certainly are. I'm not sure that something like
a small marine diesel is better simply because it's a newer model.

If I were replacing an engine in an aircraft and strapping my butt behind
it, I would prefer an engine overhauled by a good overhauler who looked at,
evaluated, and adjusted, each part individually than a brand new one from an
assembly line. I'm not sure this hold true for boats but my first
inclination if I had your engine would be to go for the overhaul.

--
Roger Long