I don't think so. It looks clean but not physically damaged. My guess is maybe that's the point where carbon got squeezed by the valve?

Got port side header pressurized with garden hose water, no leaks visible yet....

Be careful using the city water here, when I did mine the gauge read 75psi.

do not need that much.

Don't be afraid to put a little heat around suspect leak areas to simulate real world conditions.

Yeah there's a lot of pressure. I have a 45psi regulator that I used to use for flushing outboards, I'll plumb that inline again.

Does anyone have an opinion on the ding to the piston? I'm leaning towards not touching the bottom end in this thing at all. Should I attempt to clean up that ding in some way?

No sign of any header leakage with pressurization and heat...

Previous owner pulled the port header off the port motor, to replace a trim pump motor, when he reassembled he did NOT use a tailpipe gasket, instead just slathered red RTV all over the joint. Based on the soot and water on the inside of the v-band clamp, there was likely leakage at that tailpipe joint, leading to a risk of reversion. With #5 and 7 being the cylinders closest in proximity to that source of water via reversion, I think that's what led to the rusty exhaust ports on those cylinders, and the increased carbon found in those combustion chambers likely due to this as well.

No rusty exhaust ports elsewhere on either engine.

I still think I'm going to sell these CMIs and go with a set of Stainless Marine manifolds since I am a salt water boater. Eickert manifolds are NLA with no date of being available again at this time.

I use 1/16 high temperature rubber and cut the gaskets out of that. I think the vibration will cut through RTV.

Good idea too. The RTV didn't stand a chance, there was little to no bond between the header and the tailpipe. A better grade of RTV, like the "Right Stuff", might have been successful with adequate degreasing prior. The weak-sauce red stuff was a loser from day 1.