if you are talking about just the arm that is attached to the backrest, then its an easy fix. Buy 2 strips of aluminum (one for each side), 1/2 in thick, 1 1/2 wide and whatever the length is. Match it up to the plastic ones. Sand the edges to make they are not sharp. Round off the top section. Drill holes where needed to put screws back in. The whole project should not take more than an hour. The aluminum bars should cost lest than $50 US.
Or, you can just buy whole new seat, take time to remove it, fill the old holes, mount and place the new seat and drill all new holes, making sure they are sealed correctly. The cost: $500 or more, plus the time and materials to install. Good luck in your decision and project.
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