As regards the bellmouth opening that'll provide cooling air to the engine compartment (and ultimately air that'll fill the wake of the bike), I just wanted to archive this here:
AutoSpeed - Building and Testing an Airbox
"Bellmouths
The sharpness of the radius of the lip that surrounds a bellmouth is important. If it is too tight, the airflow ‘unsticks’ on the transition around the corner and the intake flow is reduced.
In Axial Flow Fans and Ducts (R. Alan Wallis, 1983, John Wiley and Sons, ISBN 0-471-87086-2) the minimum radius of the bellmouth lip is specified as being best between 0.25 and 0.3 times the diameter of the tube. So for example, a 7.5cm tube should have a bellmouth that has a radius of curvature that is 1.9 – 2.3cm. (Where did these figures come from? 7.5 x 0.25 = 1.9, and 7.5 x 0.3 = 2.3).
To turn it into data that can be more readily used, the diameter of a disc that can be nestled inside the outer face of the bellmouth lip should be 0.5 – 0.6 times the diameter of the tube. Most bellmouths are in fact smaller than this – these represent figures to be strived for."
So the curve of the bellmouth should be 0.5 to 0.6 times the inner diameter of the tube the bellmouth is feeding, to prevent air from detaching as it enters the bellmouth.