I am not quite satisfied with 'as;' but it is the nearest approach to the sense in English. 'For,' I object to; because then faith is made of positive worth, having the value of righteousness; whereas the sense is that he was holden for righteous in virtue of faith. 'For' does not go far enough as righteousness; too far as to a positive value of faith. Faith might be reckoned for righteousness, and yet the righteousness come short of what was required; whereas if it be reckoned as righteousness, then the full value of righteousness, as such, is seen: 'the man was held to have righteousness.' It is a Hebrew form. See Ps. 106.31. Gen. 15.6, where there is no preposition, makes the force of the expression plain.
'Found strength in faith' may be rather free, but 'strengthened by faith' might very easily be applied to his body, whereas it means that he was inwardly strengthened by faith. It is, I apprehend, in opposition to 'hesitating through unbelief.' Literally, it is 'gifted with strength.' Abraham was gifted with strength by faith (i.e. inwardly, in contrast with doubt).