You normaly change rollers & belt together anyway
The rollers get flat spots because they wear that way , They get dirty get slightly stuck and slide instead of rolling , This makes a tini flat spot which in time will get bigger as it will now get stuck at this point ( reving a T&G when stoped also wears them out quicker )
Belt & rollers are consumables so get changed at 6000-9000 service intervals depending on scooter
Your rollers are 15 x 12 - 9 grams , If your bike is standard then stick to standard weight , If you have a sports pipe then fitting slightly lighter one's might help
Stick with genuine yamaha belt/rollers & not the malossi rubbish .
Here's a guide on fitting them - http://beedspeed.com/html-pages/HowToFi ... eights.htm