You should be able to fix the one you have. There is a small spring that holds all the "dogs" in place. If you are careful you can remove the spring from the sprag, find where the ends of the spring are twisted together. Shorten the spring by about 1/8-1/4" (this depends on how stretched the spring is) Reinstall the spring on the sprag, then the sprag in the starter gear recess (make sure to note which side faces out before you take it out as it MUST go back in the same way, I just put a small dot on the outside with a punch lightly) Reinstall the flywheel, the alternator rotor, retorque the lock nut on the alternator (I recommend using locktite here), Bend the tabs on the nut locking washer put the cover on and your done.
Thats it in a nutshell. I do have pics of the last time I did this to my 851 if that would be helpful to you. They use the same sprag clutch. If you lean the bike on a milk crate on the right side carefully you dont even have to drain the oil from the engine. Otherwise the Sprags are about 100 bucks if memory serves me right.
Yep, Ducman is right on, its a fairly common problem too.
Just so you know I thought mine had the same problem but it turned out to be just a failing starter that eventually died. I had it rebuilt by an auto electric shop for $40 a new starter costs $560. You might want to pull the starter and check it out first before you start tearing everything else down.
i already went down that road and had the starter rebuilt a month or so ago... turns out it was only part of the problem. As long as I have the case open I think it better to just replace the bastard, but does anyone know where i can get one? my local seattle area dealers are both out.