I have use an RK 50LFO (XW ring) in a 520 for my KLX650R (enduro model), NX650, and my KLX450 and 530 (for my ZX9R and FZR1000) and never had problems with stretch or breakages. The same will be going on the KLX500 when it is finished (I have a fresh one waiting).
RK claim it to be their strongest, and I even get several from sprokets to each chain on the big bore thumpers (I know you should change it as a kit, but the chain doesn't even show any signs of stretching and the chain and rear sproket still appear new with very minimal <10%wear on the rear sproket).
Hope this helps.
TSF