I did a similar swap from a ’61 e-start to a ’57 18hp manual Johnson a few years ago, worked perfectly.
Think I had to add a bolt facing forward on the bracket to accomodate the rubber bumper for the cowl though.
Also swapped the lower forward pan from a ’58 18hp with its modification for its "stop" button. The pin-plug from the ’61 internal harness fit right into the protrusion, so I had a quick disconnect, no holes to drill in the ’57’s pan, slick.
So now I have a "hybrid" ’57-’58-’61 model.