You can figure the gross flow rate per minute requirement for a 4-stroke engine as
(1.3liter) x (8000rpm)/ 2
You can refine this a bit by multiplying by a typical  volumetric efficiency for your type of installation.

You might be able to find design guidelines for NACA ducts on the NACA report server at  http://naca.larc.nasa.gov/
The search scheme is clunky, but you will eventually find something.  keywords might be "flush" and "inlet".

One comment from experience:  A NACA duct is only useful in applications where you really don't want much air, at least not as much as you hoped, and certainly less than you expect.  And if you want any air at all, make sure the NACA duct is placed in a region with a positive pressure gradient; i.e., put it where the air sees the body as increasing in size, not constant or decreasing. I've seen lots of misplaced NACA ducts where the airflow direction is backwards.

 
 

This duct is more than big enough without aero calculations remember the size of your throttlebodies? The worst case area sizing is in first gear at full throttle (no speed) as long as your duct losses (straight and smooth not corrigated) and inlet area are bigger than the throttle body area It won't cause a huge problem relise that NACA ducts are used to conver high speed air int low speed high pressure to improve radiator core drag efficiencies if you have a look at open wheelers or planes their intakes are small relative to the surface area they feed

In David Vizard's "Tuning the A-series Engines" book he gives a graphic representation: for an engine producing 100 hp for one minute, the air volume consumed is~ 150 cubic feet (works out to a 5.3 foot cube) and 27 cubic inches of fuel (3 inch cube). Use that to figure the velocity through your intake duct and I think it will be okay- scale up if you think 100 hp is too low..

Is the duct already installed?  
I'd plumb the intake to the blunt frontal region, and use the NACA duct to provide cool after-dinner conversation.  
With some audiences the cooling effect can be considerable.