if your kitchen sink is on the other side of the wall from your bathroom you may want to check that floor as well. replacing a section of a wall is not hard if it is not a support wall and i don't know of a way you can replace the floor under that wall without taking it down. I bought a fixer upper a few years ago and have found that anything is doable. if the subfloor under the wall is not rotten then remove the toilet and take a skill saw set it to the depth of the subfloor and cut out the rotten section of floor. I hope i answered your question let me know.