But, my real question is whether it is possible or practical to just
replace the hot bus bar at the same time since the old one has heat damage
at a few places.

I had problems with a panel once. I went to a Electical shop and they had
bins full of parts salvaged from old panels. The guy rooted around and
found the exact part I needed and gave it to me.

I have experienced warm moist air from the house travelling up inside the
servace entrance conduit and then condensing when it reaches a cold spot.
The water then drips back down into the panel and causes it to fail. I
fixed the problem by sealing the bottom of the conduit where the wires enter
to prevent the warm moist air from entering.