It is probably a low battery that doesn't have the juice to spin the starter under load. IT doesn't take much to turn it when its free but the starter takes a lot of juice when it's trying to turn the engine over. It's apparent the starter works, so I'd be looking at the battery first. If the starter is turning with the sprag clutch installed, the sprag could be bad. Is it the one that came out of the bike, or did you get a new one? It's not the most reliable contraption in the world. If the rollers are slipping, you will get the results you're having.
Bare
Bare