I had to cut a square out of my wall about 5 inches or so. How do I go about plastering it up?

We had a slight mold issue in our bathroom, which I resolved by spraying the area with a bleach mixture then leaving the door or window cracked when I took a shower. No more mold! The ceiling in this bathroom is textured. Not popcorn, just what looks like a brushed based texture. It's cracking in a lot of places but isn't falling down. I could remove little bits of it with my fingernail if I wanted to but it won't come off easy. I want to prime and paint the ceiling as I'm about to paint the walls. Should I put down a "heavy" primer, then paint (hoping that the cracks will fill in) or should I put up some thin sheetrock then prime and painting. The other question is fixing a water damaged ceiling. In one of the closets the ceiling suffered some damage (it's cracked down the lath/plaster/whatever) really bad. I was going to cut some sheetrock to fit this area then prime and paint. The problem that caused the leak has been fixed. Would putting up some drywall be ok, or should I remove the old stuff first? This area isn't too wide getting up there to hammer and chisel the ceiling would be very tight unless I can drop down to about 100#. :)