Replacing roof vent on 1976 Starcraft Starmaster 6 with aluminum patch.

I haven't been the biggest fan of roof mounted vents on any RV really. I guess I could see it on hard walled campers more though. Anyway the roof vent on my Starcraft is starting to leak. Cracks along the flange and cracked dome. Darn aged plastic. I found another on amazon. Unfortunately it is another plastic one. The vent is a smaller 9 inch vent rather than aluminum which no one makes in metal. I'm not really enjoying the idea of cutting the hole bigger either. What if I just placed an aluminum plate in its place and sealed it up instead? The vent is over the galley stove I don't really see the advantage of it as I could just open the flap that is above the stove in the wall. My Parents Starcraft doesn't even have any evidence of one. So it seems not all models even had it. I just don't trust the plastic of today. Let me know what you think.