I don't know if anyone else noticed this but. you said that the bike was modified by the previous owner. In my experience, when you change from a 525 sprocket to a 520 sprocket, you have to use a washer that is provided in the kit to keep things tight and as you stated, perhaps the previous owner didn't bend the tab on the washer up after he tightened the nut.
Sorry to be a wet blanket but I don't want you to get your hopes up.
