I had this thought at a gas station so it may be good or may be bad. I think the magus's striking spell should all be based off of their martial attack roll. You would have to take away the thing about not losing the spell if you miss with your martial strike.
So for using attack roll spell with striking spell I would just transfer the success degree of the martial strike to the degree of success of the spell attack roll. So a fail or crit fail on the martial roll means nothing happens, but a success equals a success at the spell attack and so on with the crit.
For save spells with striking spells I would still only have a single martial attack roll, but inverse the degree of success for the save. So a crit fail on the martial attack equals a crit success, a fail equals a success, success equals a fail, and crit success on the martial attack equals a crit fail on the save.
Any comments?