Check easy stuff first. If the starter motor spins, and it sounds like it’s spinning fast, it’s probably good. Make sure it’s fully inserted into the housing, and has 2 bolts on the ass end securing it to the cases. Second, there are 3 covers on the clutch side (right side if you’re sitting on the bike) the big one covers the clutch, the medium one is at the crank, the small one is where the starter clutch is. Take that one carefully off. There’s a gear in there, a center hub pin, and some washers. Either get the service manual, or just look at a parts diagram. Are all the parts in there and in the correct locations? Lastly, you might wind up taking the clutch cover off, draining the oil, and making sure dude put it all together properly. It does sound like something ain’t right with the starter gear though. Please post what you find!