If you have a crawl space, might as well remove part of the old plumbing and adapt back to it. Running pvc up will make putting a new flange on much easier and if done properly will insure that your stool won't rock back and forth. Looking at the old lead, may have a hard time insuring that when you put the new flange on that your stool won't rock unless you shim it. If you don't want to go through the trouble of replumbing under the house. Get a repair flange that will fit around the lead and go from there.