Unfortunately there aren't any replacements for just the hinges. You have to purchase the entire seat.
American RV Company has Magnum Opus replacement seats in three different colors, Bone, Ebony and Platinum. Not cheap!
Don't know what color you are looking for but here are the part numbers for white and bone.
385344013 - White
385344014 - Bone
These may work also.
385310857 - White
385310858 - Bone
Can you post a photo of exactly what is broken with your current hinges and possibly someone can come up with a solution for you.
I have had a Magnum Opus since 2010 well over 6 years and those hinges are quite strong. May I ask how yours were broken?
Dr4Film ----- Richard