As a PPL and later a CPL/FI, I have had some experience of VW aero engines. Basically a Beetle motor with dual ignition and two spark plugs per cylinder. There are also some chaps who have successfully 'halved' the VW flat 4 and used the resulting 2 cylinder motor in very lightweight aircraft.
We have had a few Beetles in the family, a 1300, 1600GT and a 1303 super beetle, and a bay window camper, so that along with John Muirs excellent book for the 'How To Keep Your Volkswagen Alive, A Manual of Step By Step Procedures For The Compleat Idiot', means I have a soft spot for this engine.
I always remember reading the Fabulous Furry Freak Brothers comics, and the 3 hippies in their camper speeding away from the Narcs with the sound of 'Vroooooom-tweetle, Rooooaaaaarrrr-tweetle'
The most perfect literary representation of a VW flat four I've ever heard!