I'm rebuilding an electric scooter with mid-engine configuration. I can tell you it's not worth the effort.
Hub motor is the way to go.
3kW sees a little on the weak side. But for a light motorcycle, it's probably ok. (Something that had 80-125cc in its previous life will run ok with that)
In Europe light motorbikes are allowed up to 11kW and mopeds up to 4kW, you got some room there.
But engine power isn't usually a significant limiting factor. Just select one that has the right speed and voltage rating for your power system.
What will likely limit you more (especially in acceleration) is your motor controller. Overspec that by a lot if you can and allow high peak currents.
PS: Your bike weighs 20kg with batteries? wow, that is light.