Stanley doors becoming difficult to operate

I had 2 Stanley 36X96 Monarch Prism 321 mirrored bifold doors installed in a closet in my home in August, 1996. The bifold doors are becoming very difficult to fold open and straighten out to close. The track wheel is broken on the one door and the same is wearing thin on the other door. Can I replace these track roller wheels? The item number is 4465. How would I do it if I could replace the missing and worn parts?