The 170/175 (no difference in length, just a new decal) always had trapezoid fiberglass stringers filled with foam. I think about the late 90's they started using poured material in the transoms of the entire Aquasport lineup. The deck is cored with end-grain balsa, the casting deck is cored with the same or plywood, The hatches are balsa cored.
Fuel tanks - aluminum was the norm until maybe the mid '90s, when they changed to poly. The aluminum tanks do get corrosion, depending on how they were installed, which ends up being tiny holes. If installed correctly and allowing airflow around the tank, can last a very long time. Some people coat them before they are installed.
Outboards - the 170/175, regardless of the year is sensitive to weight in the aft. In the old days the 90 or 115 were pretty light, the old merc I had on mine had the twist transom clamps on it. These babies were thin, light and powerful - perfect for many of the smaller models of Aquasport. 320lbs seems to be about the limit before your scuppers go under with just engine weight. The old motors had a different mounting pattern, or in my case, no mounting patterns to drill. My '95 90 Johnson is about 320 lbs.
