Suzuki bandit possibly the best bike I have ridden, sturdy reliable and nippy.
Triumph speed triple possibly the bast naked bike you can get and performs like a four cylinder bike.
BMW's are ok bit their engines and shape are not the best.
As for a Harley, hmm all noise and no go!
Leg exercises really are all you can do as you cant train just one are as you will have an imbalance like that character from the terrible film lady in the water.
leg press, leg extensions, reverse leg curls, lunges
Cardio, just do cardio, get that calorie deficit going and stock up on vegetables, lean meat, water and stay away from fruit juices and high fat foods.
Attending a training class will help as it will push you harder.
GH for kids!? I don't think that's a good idea especially when he has not fully developed yet.
GH at this stage may have an adverse effect in the long term such as a lower testosterone production which can lead to further problems.