[jclark003]

Pretty sure the exhaust manifolds are inside the coolant loop on those engines and not subject to sea water corrosion. As far as the risers go if you want to clean them you can try a radiator shop. Ive had good luck with the shop over by west marine in Holland. Otherwise you may want to look into replacing them as a preferred option. Crusader offered a "heated riser" option which I think runs coolant through a half raiser, with raw water exiting the top. so check which version you have before ordering any new parts.

A good radiator shop can also clean your hear exchanger core if needed. There are products that will chemically clean them as well.

Good luck