Putting in larger windows weakens the structure of the
cabin top, and if the
boat is used in the open ocean, therefore vulnerable to bashing in the side of the
cabin top by a breaking wave. If it is always used only in sheltered waters and nice
weather, it may not be a problem.
We had a flush
deck boat that was very cavy below decks. What helped immensely was painting the overhead bright sparkling white. If
paint works for you, I'd do it. It doesn't take much.
As to the cracked port hole "window". Surely a glass
cutter could cut you a new one, or if it is
acrylic, or
lexan (polycarbonate) then a plastic shop.
Ann