I would check if all of the linkages related to the timing are properly tensioned, as I would think the timing for running clockwise would be different to counterclockwise. Also, if this motor has a rubber water pump impeller, it would be a good idea to check if any vanes flipped backwards. Personally, I think spark plus are thought to be the culprit of an issue waaaaay more often then they actually are the issue, so if I were you, I’d first look elsewhere for the cause of the problem.
