part that fits into the shoe

Hello!

It sounds like you may be referring to the pivot shoe's cam. The cam can't be replaced but the pivot shoe itself can. Show us a picture of an undamaged pivot shoe removed from the track and we can narrow down some options from there. I also related some videos on this page that you may find helpful. 

Thanks for the pictures! This isn't the cam at all, it actually looks like a plastic pivot bar. Start by taking a look at our 26-400. Compare the dimensions and pictures to an undamaged pivot bar to see if this matches what you need.