I'm pretty confident that one of those Chinese import air cooled engines will suck in sufficient air with it's built in scavenger fan and vent it out the other side . If there is sufficient opening ....and there will be ... under the front of the fairing.The engine will be open to the front wheel enclosure.
Diesel Engine V Twin Yangma | eBay
The calculator at the top of the page gives my design about 1.5 litres / 100 km if the cd is .55 which MIGHT be possible to achieve .A cd of .65 gives 1.77litres / 100km with a 5hp requirement all at 50mph or 80 kmph.
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
Allert's bike would easily achieve his goal of 1litre / 100km if it used a diesel. By my calculation reworked from the data he supplies I calculate he would achieve .8litres per 100km or 300mpg US.