Stepped keys are a standard engineering solution to mismatched parts, Keith, so what you have done is quite legitimate. Occasionally they are made stepped both sides, but it is far more common to only have a step on one side, as you have done. However it should not have been necessary: I think it unlikely that the mower had a stepped key when it left the factory. Most likely it had some kind of British engine originally, and the Briggs and Stratton was fitted as a replacement. Provided you have achieved a good firm fit in both keyways, it should now be effective and reliable for as long as the mower lasts.
