Is scale excessive? - hard to tell from pics. They do look like they took some water at some time in their life
You do not state whether you run in fresh or salt water. What do the risers look like? How about the riser gaskets? Water could have leaked down from risers to give some rusting. The boat could have take water in from hard stops too. What does the exhaust flapper look like?
Did you pressure test them? Make a rig to accept garden-hose fitting, and block other side, but allow for bleeding air out - maybe screw a small valve into this end. Then make a plate with some gasket material to block riser surface - or use riser with a full gasket. Hook your city water garden hose, that supplies about 55 psi, out of the faucet to manifold. Let is sit there for a while, and carefully look for leaks. You might use a flashlight to see in the exhaust ports. With water pressure on manifold, also lightly tap with hammer around on casting to see if you loosen up some scale to cause leaking.
If they hold water back, I'd clean them up, scrape gasket surfaces, then make determination. If they don't hold water, scrap them.