The flange should allow for some rotating to place the stool in the correct position, unless it was installed so that the flange attachment slots are positioned incorrectly. In that case, you will need to remove the pipe to the tank and reinstall the mounting flange in the correct position.
One side of my Thetford Aquamagic lid hinge was broken. I tried my whole arsenal of adhesive on that plastic and nothing seemed to hold, long-term. I finally drilled small holes in each piece and installed two double-pointed shafts that were heated and fused to the parts. That, along with epoxy, is proving to work. The OP looks to need the whole enchilada, EBay is going to be the cheapest, I would think.
