Sorry but not, as a game master you decide/improvise the character stats. In general, I'd suggest you start by thinking the possessed NPC strengths. For instance, a monk or a priest might have more spirit because of their practices, or a samurai could have a higher physical bonus due to their martial discipline. Once you've pinpointed that strength, give them a +2 bonus an the rest of it a +1 ,0 or even -1 , but, of course, talk with the players at the table if something doesn't quite fit with the character.
I'd also recommend asking questions or letting the players answer them, such as, "Why is this person here?" or ""What did this person lose when they were possessed?" an d once they're possessed, you can ask the players for more details about the possessed individual. Remember, they have access not only the body but to their mind and spirit to(And it could be a good opportunity for a Will stat check). Take note of the players' ideas and proposals and collectively decide what works best to enhance the fiction. I hope these tips help you run the game smoothly, and, of course, give me the scoop on how it all went!