First, the weight. The boat should sit on its waterline, which means the line between the red and white on the hull. If its lighter, add weight on the centerline (fishing weights, lead shot from the gun store) until it DOES sit precisely on that line. If heavier, remove what you can until it sits there. It looks like it was painted in the right place, so try to get there. Real boats sit right on that line.
I get the idea that you don't have a ton of money. So:
Power - Lindberg NEVER provides enough power. MACK used to provide a power package for this boat that would get it on a nice plane. I would dump the single motor/gearbox setup (and no, I wouldn't trust that gearbox either) and change to twin motors somehow. Check Radio Shack for their 6v motors - cheap. Mount the motors however you can (scrap plastic and rubber bands if you've gotta) and use model airplane fuel tubing to attach the shafts to the motors...
If you can't change the motors, then ensure you have as much rudder throw as possible by changing arm lengths on the servo and rudders. You need POWER and rudder throw to turn that thing - the prop's thrust against the rudders is what turns it....good luck. And if you wanna sell it, call me
