He modified Madara to be stronger than when he was in his prime. It's called necromancy, and Kabuto was very adept at using dead bodies for power much like his teacher.I do read the manga, when kabuto modified his body he just enhanced with hashi's cells. When you use edo tensei you can't revive someone from an earlier point in their life! You can only revive the at the age when they died. Because a madara was still alive in his prime and he didn't have rinnegan back then. So this is indeed a plothole. Madara never died at that age, so how is it he revived him as such & with rinnegan something he didn't aqquire until he was old?