Since your heads are cooled by the closed system, they should be okay. The exhaust ports are always going to be hot and burn off the paint. If your manifolds are totally shot, you might have water from the cooling jacket leaking internally into the exhaust port. You can get water to the exhaust valves and into the cylinder.
Replacement manifolds are available online through most marine vendors. They aren't going to say Indmar on the side anymore, but at least they are available.
The good news is: Once you turn off the sauna, I think your vapor lock problems will be fixed.
The bad news is: Those manifold bolts don't look like they will want to come out very easily.
'96 ProStar 205 SD LT-1