Mild overhaul and then repower, Cat 3208 to ?

I just bought a small passenger ferry 13,5 x 3,5 meters which is powered by a Cat 3208 375 hp. Circa 12 knots @2100 rpm and just shy of 16 @2500.

The boat hasn't been maintained very well and I'm most likely going to give the engine a light overhaul so that I can trust it to last for a few years (rings, valves, bearings etc). I can either do this by pulling the 375 hp engine and sending it away or I can overhaul a 435 hp version that I can get cheap and then simply do a quick swap.
If I'm not mistaken the 435s doesn't have the best reputation. But will it last just as good as a 375 as long as I'm not using the extra power (I won't change gearing or prop)? Or do the 435s have other issues?

I'll run the boat for one or two seasons and if it works well I'll most likely fit a new powertrain that can give me a cruise speed of 18-20 knots (if I'm in a hurry).
I'm using FPT in several other boats and they work very well. I would have liked to fit the 16 liter 815 hp medium duty version but it won't fit unless I stretch the boat which is not too big of a deal but I'd gladly avoid it.
What options for a good 3208 replacement with at least 600-700 commercially rated hp are there? It needs to have about the same footprint as the 3208 (and of course I'll replace the tranny, shaft and prop to match the extra power). Width isn't an issue but length is.

 

I would recommend the Cummins QSM11, though looking at the spec sheet it seems it might be longer than the 3208 and maybe that's a dealbreaker.

The Cummins QSM11 Marine Engine Story https://www.sbmar.com/articles/the-cummins-qsm11-marine-engine-story/

 

They got a bad rap because they didn't have a liner, other than that it isn't a wild amount of hp per liter even for it's vintage. We had a lot of them up here over the years, they were a relatively low profile motors and very popular in our shallow draft small fishing boats.

They posed a challenge to repower on the straight angle boats as most long stroke inline 6 motors have much deeper proportions. The down angle boats just required the motor to come forward and up. Actually passed on a nice boat a few years back that had worn out 3208s and a hull configuration that would need either some serious surgery or rebuilding the older tech. Seems like the most popular swap is the 6 cta. It's less displacement but higher hp. It's also a lot easier to stuff in a 3208 space than the 13-16 liter mains. It also usually stays within the torque saftey factor of most drivetrains negating the need for bigger shafts and bearings.

Once you step up to the 13 liter class, it's all just trade offs. John deere has lost its diy fix luster and popularity with it, seems to still be a decent motor but struggles in the high speed applications. Scania was the poison I picked, do believe it's the better mousetrap but when it's time to buy parts.... it makes me crazy. The Volvo has all the intangible frustrations of scania but a slightly less convenient dpec sheet. Man.... well I don't catch enough fish to buy or maintain one. The fpt seems popular, time will tell if they support them like cat/cummins or like volvo/scania/John deere. Don't see enough miles on the fpt yet to have a strong opinion.

The not cat "cats" have enough miles, and the general consensus isn't giving anyone the warm fuzzies. Have seen more than a few pulled long before normal swaps would occur. On the cummins side, they dropped wet turbo aftercooling between the cta and the kta both the 11 and 15 liter cummins offerings are dry turbo. That has made them very unpopular in our fleet.

I whine about my scania a lot, but it mostly stems from just the times we've needed parts. All in the engine has been good, just compared to my previous legacy cat and Detroit motors parts are hard to get and super expensive. That said, I'd maybe gamble on the fpt if I had to repower.

 

How many hours on the 3208?
If it works, don't fix it.

Talk of a bigger engine and extending the boat makes me wonder why you bought the boat in the first place.
More importantly, what do you plan to do with the boat?

EDIT: How's your electric taxi boat working out?