I think the stretch partly comes from the small front sprocket. ive been looking around and i can so far only find 520, and im not ready for 520 yest.
it dyno´d 152/123 on a happy dyno so 525 is what i look for. i bought a cheap 530 xring for thge break in of the engine and it stretched alot in the first 800km. 700km break in, oilchange and 100 semi rough kilometers.