Chop a hole in the wall, fit a new soil outlet into the stack. Fill the old vertical pipe up with cement and point smooth in the chamber where it joins the rest. Just done one last week. It's the only really satisfactory way to get a close coupled WC in. Your only cheap and easy option is to install a low level pan and cistern instead of close coupled.
Or: stud off the back wall to suit a Close coupled.
If you do it full height it will not notice at all (unless there’s a window!)
Or my preference is usually 1000 to 1200mm high (depending on height of room) finishing off with a useful shelf.