Unless the belt is frayed, I wouldn't change it.
I used to have the SIP version of this, and it served me well for many years.
The belt tension varies depending on the speed setting on these "Variomatic" lathes.
The one time I had trouble with it (starter capacitor), they sent me a free belt as well!
Bu##er to change, and no difference in size to the old one!:nonono: