Giuseppe, a little secret. What we use are sewing machine belts. Although I just bought an Aquacraft Grimracer starting belt for my JAE .12, I've also picked up used belts from sewing machine sales and repair shops. Most dealers will replace lightly used belts on trade-ins and repairs and put them away. I got a handful of them for a song.
Take the boat with you and after talking ot the person that does the repair work, show him what you're going to use them on. If they are anything like the guy I got mine from, he got really interested. I think that's partially why I got such a great deal on them.