IMO
Unless you have noticed degrading performance, let it be. Enjoy your riding !
A used part is a proven part.
IF you are planning an extended tour, far away from home, by all means, replace the belt with new and ride it for a week or so, around home so it can prove itself.
Nothing wrong with having a spare in it's plastic bag, hanging up on a nail out in the garage. Price will only go up tomorrow...
redk