We don't know the tonal centre but who cares. Score what we do have yourself, what key sig would you use?
I would use C.
Re-reading the original post I see the poster knew this anyway so I take your point that we don't have enough to answer his question of where the tonal centre is. I would still say C though, we don't finish on G, that's the last chord. Em would end it - what's Em anyway but C maj7 without it's bass note?