I'm no Selectric repairman, but a peek inside a
nice
wide-carriage Selectric III that I've got stashed away looks
like replacement of said belt is a major operation and probably
not worth the trouble.
You know the entire outer shell drops away with two lever pulls, right?
I've removed the main drive engine from a Selectric (where the
belt passes) and would say it's worth the effort (not much effort)
compared to the incomprehensible geometry of it's lever upon
lever system for translating key to typehead. Aside: someday
I would like to hear said system described by someone. I've
found it fascinating.
John A.