Broken piece, need replacement

This part is called a pivot bar. I can't say for sure what yours is since most of it is missing, but start by looking at our 26-369, 26-400, and 26-412. Your next step will be to get a hold of an undamaged version of your pivot bar and carefully compare its dimensions to each of these three parts. If none match, you'll have to tell me exactly how yours differs so that we can find something better.