So here is the thing the ghost lingered protecting his own family grave and had his friend upon his death bestow magic on his corpse, that would trigger when the tombe would be invaded and the corpse would become undead, a zombie. On which the ghost would possess his own body. Since the ghost can speak and remember all and now has a body with the decaying muscle memory of his former life. could it be possible to use his spells now while possessing his own body albeit decaying? I could not find a clear answer on this or if it was possible in normal dnd rules to do this.

RAW Possession is used on one humanoid that the ghost can see within 5 feet of it, which exclude zombie being undead.

But as DM you can always rule it works for your story.

thanks ;-)

Yep, you can give it whatever spells you want. It’s your game and monsters don’t have to follow PC rules. Many DM’s modify their monsters as needed.