All Hassans, at least from what I heard, have to completely give up on their identity to become the mythical old man of the mountain. Some one even having a partial identity is slightly lore breaking. 

Mind's Eye is a ability the inherently purposes that the applicant in question already has clarity, and prudential awareness of their surroundings to assess what is happening around them, and react accordingly. Further calming of the mind would prove redundant to the skill at hand. It's like taking a already sharp sword, and sharpening it further just to say its that sharp.

Kojiro Sasaki's presence concealment is E Rank, or roughly around there. He has little to no real hiding ability on par with Hassan taking something like that from Kojiro and putting it on an assassin who already exceeds at stealth makes it more power than it was intended on that specific character.

Okita's skills would probably drop in rank around strength but have a higher speed. She'd be around roughly C presence concealment as she is a member of the famous japanese shinsengumi (spelling not included). She would fair well in high speed attacks not too different from Saber but unlike Saber would probably focus more on quick strikes then hiding as Saber is the best class as it is. Saber Okita is more of a dodge tank since her health is trash but is fast. Assassin Okita would probably have something closer linked to the traditional shinsengumi.


Slight edit: Said clarity of mind twice.