Yeah, I think the petcock being left on was coincidental. Even twisting the throttle shouldn't cause a problem. The starter is the culprit here and the one way clutch should never let this happen. I did a used motor for one guy with the same issue. The motors usually come from wrecked bikes. Check Ebay, the motors are out there and are usually complete with parts you don't need which will allow you to sell some stuff off to recoup a portion of your costs. It is a beautiful bike and must be fixed.
