For comparison, I will use a 170 MONTAUK boat that was
listed recently and sold in a very short time for $35,000. That boat was newer, 2015, but its engine was older, also 2015.
As far as the boat, as long as the condition of the 2005 boat is excellent, I do not see a great difference in value compared to the comparison 2015 boat. For a boat from Connecticut, I would expect it was used in saltwater, which would tend to reduce the value compared to a freshwater-only boat. The original engine on a 2005 might have been an older two-stroke-power-cycle engine, and the owner might have wanted a newer engine. Or maybe the boat was used very frequently, and the old engine just wore out.
Having a practically new 90-HP engine on the boat you are asking about is a very nice addition to the package. If someone just put a rather expensive new engine on the boat, they probably invested at least $10,000 to $12,000. That prior owner is probably expecting he should get most of that money back when he sells because of the very low runtime on the engine, but I think in the real world, that is not going to happen. As soon as you buy a new engine at retail and have it installed, the resale value of that engine is going to decrease from whatever you had to pay for it--unless there were to be some crazy market conditions like a huge shortage of engines.
Again using the 2015 170 MONTAUK sale as a comparable, I think a similar $35,000 price would be very attractive, but the almost-new engine will probably increase the price by many thousands of dollars, and the seller might be holding out for more return on his recent investment in the engine.
In addition to the boat and engine, there can be a lot of other elements that add value, such as Bimini tops, sun tops, console covers, mooring covers, trailering covers. To have $4,000 of nice canvas on a 170 MONTAUK would not be vary hard to imagine. The electronics on the boat could also be more elaborate than a single 9-inch multi-function display. And comfort and accessory items such as enhanced seat cushions, bow cushions, better seats, coolers, rod holders, anchor tackle, fuel tanks, or a VHF Marine Band radiotelephone will also be adding value.
The essential element of all of this is the condition. What some fellow thinks is excellent might only be "good" for the next guy. You should look at the boat in person and give it a careful visual inspection. The seller's circumstances also can be an influence. Maybe the boat was bought with a loan and the seller is getting tired of making payments on a boat he does not use. Or he might be willing to wait a long time to get a higher price.
This time of year is usually a buyer's market, as sellers may be facing paying for winter storage, which might cost them a bundle if they can't keep it on their own property.
I would get a firm quote on a new 170 MONTAUK, and use that as a reference for pricing. Let's say you could buy a new one for $50,000. Then the value of the 2005 is probably somewhere between $35,000 and $50,000.