Suggest u send picture of the “water jacket” that accepts the exhaust and channels water that cools exhaust. Does it have Gustovson stamped on the casting?
Your heat exchanger looks a lot like our 1979-1980 Ford with Swedish Gustovson (Gustovsen?) marinization.
We have cleaned and re-assembled our water cooler. If your’s is Gustovson, what has failed? As I understand it, Gustovson has stopped making these parts? They hard to find.
Some of the o rings can be replaced with Yamaha o rings. Zincs are readily found in US (I can dig out part numbers if you need that)
In US, seemingly the only option once system has failed, is convert to Lehman. Costly, but solid.
American Diesel has a tranny oil cooler, but I guess, since an independent unit, others would work.
What has failed or causing concern?