Designing Kitchens and Bathrooms - Subflooring Replacement

Our toilet sits at an angle. When we peeled up the linoleum around it, there was a hard wood floor that had rotted away. The subfloor is probably rotted as well. I've looked at instructions for replacing the toilet itself, and that seems simple enough. However, how difficult will it be to replace the original subflooring, which is planks, with plywood? Will I need to replace the sewer pipe (this thing is really at an angle)? Will I need to pull the shower too? Should I hire out for this?

Thanks.

PS. Upon closer examination, it looks like the people before us replaced some of the floor or reinforced it once already.

What you would need t do is to replace the old subfloor planks to the nearest floor joist. Cut te old boards to the center of the joist and then put your patch in place. What kind of piping is going to the toliet. It may very well be that when you replace the subfloor, the lean will be gone. When you replace the subfloor, determine where the opening will be in the patch, and then cut it down the middle in the centerline of that hole. The hole is going to have to be fairly tight to the piping of the tolit bend. You probably will want to install some bracing under the patch to support it. Like this

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You would put two support that are perpendiculr to the floor joists, and then put two supports that are parallel and tight up to the drain of the toilet.