I have a bad copper drain pipe under kitchen sink that goes into wall then down to crawl space and is soldered to copper pipe in crawl.Can I replace by putting hole in floor and basically go down vertical and connect to copper pipe w/ fernco after cutting copper pipe ? Or should I cut wall out ?

Yes, you can. Problem is that you will need to reconnect vent inside the wall. So you will have to cut the wall inside the cabinet to get to it. See the sketch...

Can I replace by putting hole in floor and basically go down vertical and connect to copper pipe w/ fernco after cutting copper pipe ? No you can't. That wouldbuild a "S" trap.
Milo's correct, You have a sanitary tee and a vent inside the wall you have to connect to.
When I was faced with the same problem some years back I was looking at the hassle of removing the sink and cabinets to open up the wall.
I found a easier, more simple way to replace the bad drain line in the wall.
My kitchen sets on a outside cement block wall. I opened up the outside block and replaced the pipe from the trap to the vent with PVC.
I then packed the hole with paper and rags, covered the hole with hardware cloth,(screening) and patched the hole with cement. When the patch cured I painted it white to match the wall and my repair was complete without tearing up my kitchen.
You might look into opening up the outside wall instead of the inside one.
Good luck, Tom