I have a rather odd ball situation with a 1/2 bath on the ground floor of our townhouse. Recently removing the vynl to replace with tile I discovered the toilet leaking very badly. After removing it I discovered that it was bolted to the cement slab with no flange. Worse yet the waste pipe measures 2 3/4 inches inside diameter, is set in the concrete slab flush with the floor no room to put a collar around the outer pipe and too small inside diameter for inside the pipe. Any ideas other than selling the townhouse as is.
You will need to chisel out the slab from around to riser to the toilet to give yourself enough room to insert a hubless connector to connect the riser to a proper flange. You may have to chip away a bit of concrete to accomplish this, and you may have to cut the riser to allow proper placement of the flange at just above the floor level. Special pipe-cutting tools are available for this latter procedure, but they are costly. You should be able to hire a competent plumber for less than the cost and aggravation of doing the job yourself.