I just had the exact same problem with my OS65LA... in fact there was so much play that the connecting rod was rubbing against the inside of the crankcase causing a very noticable clicking noise.

As the engine would attempt to fire the whole thing would just seize up and stop. I took off the back crank case cover plate and noticed that some actual gouges had been taken out of the inside of the crankcase as the engine attempted to fire.

To solve this, I replaced the crankcase cover plate and made sure it was on very tight which did limited the amount of play in the crankshaft... I will try and start it up tomorrow and see if that solves my problem.. if not I think I will send this back and get a replacement...