If one is certain he missed a part of the body whilst preforming ghusl does he have to re preform ghusl or can he just wash that body part? If yes does this ruling have a time limit? Can he rewash that body part a day after his initial ghusl?

No need to re perform ghusl again. Just wash that part, regardless of time, but he need to perform wudu and repeat the prayers which he prayed during this period