Thetford toilet seat

Would anyone have an idea how to remove the Thetford toilet seat? Replacement seat/cover assemblies are at least $130 and the underside of ours is nasty from the previous owner (wife is grossed out), so thought I'd scuff and repaint it. The exploded view shows a pin on each side that runs through the cover into the seat and with a retainer clip of some sort. I don't see a circlip, etc. where it's supposed to be, but it's a little corroded in there. There appears to be a collar there instead. This toilet looks delicate and I don't want to mess it up, especially with the cost of parts. Also, if anyone has a solution for removing p stains, I'll try that first. Thanks for any help.