All the rejuvenating stuff will not work or will not last long .

If you have some VCR belts around that will fit around the hard idler .
A real thin one the same width would be deal but 1 or 2 of the square
ones will work .

Stretch or place the belt-s around the idler and run a bead of superglue
around to hold it on . mount the idler in a drill and run it on
sandpaper .
If you are using square belt-s you shound sand it down in the drill
first to take some off so the finished diameter is closer .

This works for me 100%