If one davened Shacharis without having kavana in Avos or Modim, and thus was not yotzi, the halacha is that nowadays one would not repeat SE, because when he repeats he also won’t have kavana.
May, however, one eat after this ‘Shacharis’ even though it is forbidden to eat before davening, and his davening was not valid?
Although technically he has to daven again, however if he didn’t have kavana it is still considered as if he davened without kavana
Shut Eretz Tzvi 22 D”H Omnum, from Iglei Tal, Eishei Yisroel 11 ftnt. 17.