Fitted a 6hp auxilliary outboard to a Norman I had a few years ago.
It was a longshaft because of the distance from the trapezium mounting to the prop cavitation plate needing to be about level with the bottom of the hull.
There were a few difficulties,first the boat needs to be out of the water,the trapezium mount need to be the correct strength for the outboard.Some makers quote for example,suitable for 10hp two stroke or only 6hp for a four stroke.
The position of the mount is quite critical in that the prop needs to be deep enough,and when the outboard is not in use,you don't want the prop dragging in the water.
The transom will quite likely need a large piece of ply on the inside with the trapezium bolted through with large washers to spread the load.On mine the transom flexed under power untill I fitted the ply.
Because of the angle of the tiller,even a telescopic tiller extension was too steep an angle for effective control,so a "swan neck"tiller will need to be fabricated,along with a longer gear lever.
It was a long frustrating job,and and really of little benefit,except as an emergency engine.
Looking at your boat photo,the towel rail on the transom would have to come off to fit a trapezium mount.
Would it be simpler to have say a 5mm steel plate about 10"×5" welded to the curved bit of the "towel rail"and mount an ex longshaft or perhaps a standard longshaft would do.
You would only then need an ex long tiller extension and gear lever extension.
It has occured to me that it may be cheaper (unless your engine is complely goosed) and certainly less work to have your engine repaired.