First, (Archie), they don't use bronze because bronze is an alloy of copper with other metals, usually tin. And copper is an extremely reactive metal. If you put a bronze elbow in a steel exhaust, they will form what is called a "galvanic pair". It means there will be tiny amounts of electricity between them and they will corrode VERY quickly. Add to that salt water or salty moisture and the steel around that bronze elbow will rot like cardboard. You could make the whole exhaust out of bronze but then it would be expensive (copper is much more expensive than steel) AND you would then have to deal with galvanic corrosion between the engine block and the exhaust. Not good.

And second, (Dad), DO NOT improve such important things as generator. If they used a cast iron part, they had good reason for it. I suspect they did because cast iron is a good heat insulator and keeps surrounding parts cooler than steel. It is very important in a contained diesel engine. Remember diesel exhaust runs 200 deg C hotter than petrol. Order the cast iron part and leave it be.