I would bet money the crank is bent. Have seen many, many, many of them in my 50 years of repairing mowers. I used to straighten them. Have a jig to do it. Stopped doing it many years ago though when the society got so sue happy. Nobody that I know of in the state will do it for a customer anymore. I have straightened some that were bent so badly that the engine wouldn't even turn, and never had one break for me while straightening or in use afterwards. I still have the jig, and use it to at least partially straighten the crank if replacing it, because if bent to bad the sump won't come off the crank. If you can do the work yourself, parts probably not to expensive, but if you gotta pay professional labor rates, may not be feasible.