As a rule motorcycle starters connect directly to a gear or through a chain that will drive that gear to turn the engine. This gear the starter connects to, is a one way clutch, in other words it will only lock in the direction the starter turns it, and it will turn freely in the other direction.
If you are positive that the starter is turnning, this one way clutch gear would be the thing to check. Usually this envolves removing one of the engine side covers to get to it. With the starter removed , you can check it by spinning the gear in both directions. If it spins in both directions is not good! Also, If you just remove only the side cover you can see if by turning the starter the gear spins. If the gear turns without engage the crank then you will know that the the one way clutch gear is no good as well. (do not run the engine, just spin it and turn it off, it's only to check.) Be known that, I do not know the configuration of this motorcycle. However, the above info is true to most motorcycles.
Hope it helps you!
Ferni