Boy, do I ever feel stoopid today! The ’58 Sportwin does share the same parts as the smaller ’58 Fleetwin and Fisherman motors so the ’57 Sportwin steering handle would not be a direct bolt on as I’m sure the bolt pattern for the ’57 steering handle is larger, same as the 18’s. This should help narrow your search down for the correct part.
Will the ’57 steering handle fit the ’58? No. Can it be made to fit? Possibly, but probably not worth the effort.
Pass the crow Jacques.