Shin Akuma cannot use the Shun Goku Satsu since it takes the opponent to hell n Scorpion gets stronger in hell, n he's also an undead. Scorpion's no pushover, but Shin Akuma will be able to take this. If its Akuma in the fight and not Shin Akuma, then Scorp can win...he has his swords, that rope-thing of his, plus he becomes stronger in Deception.
And there's always the chance that he can return coz like i said, he's an undead.
It's a tossup... Scorpion has displayed numerous times some pretty good capabilities, even including Ressurection, and is a vicious fighter to boot, Swords, Barbed grappling hooks, Flaming breath, Ability to manipulate hellfire among other abilities.
Akuma on the other hand, demonstrates enough strength to shatter massive natural formations, superior ki control, and inhuman capabilities like surviving on the deep sea floor without breathing or pressure protection. Plus, Akuma demonstrates the ability to generate far more energy than most other fighters, making him a near DBZ level character.