Small waypoint navigation drone

I was interested in modifying my hubsan x4 Drone so that it could waypoint navigate using gps. I’m told, and I can see why, it would be difficult since the gps module and flight controller boards would be too large/heavy for the hubsan x4 motors.

I also understand that ardupilot moved away from Arduino because it basically outgrew it’s capacities.

So what would be the best way to build the simplest Drone and smallest Drone possible me for the hubsan x4 motors I have and give it gps navigation?