Buy part skeg number 859447A-1 from Mercury and have it welded on.
Someone remember this Mercury part number for future references. If you are doing a Pro XS case, then you will have to find a back case and cut the skeg off. They do not have those as a part item.
Cut the skeg yourself in a reverse and sideways Z pattern, which give the Z notches for the new part to lock into and not depend on the weld only.
The welding is done is short slow welds, that allow the heat to dissipate. The oil and gearcase is left in tact, and can be left on the engine if you have a shop large enough and doesn’t mind.
Then you grind the welds, sand and fill the crevasses, prime and paint.
Overall a couple of hundred bucks on the outside.

BCB