There are a couple things that can be used to judge the "truth factor" engine builders claims but the most important thing to consider is takeoff torque and secondly horsepower.  If we look to the Continental O-200 as being the minimum for the Vision then we can do some comparisons for honesty.  First any other engine must have as much torque in order to get it going.  At takeoff the O-200 can produce it's 100 hp at 2550 RPM    Torque = hp*5252/rpm   so the torque of the little Continental is 205 ft lbs.    A VW engine usually peaks in torque well over 3000 rpm.  That means that it would need to produce 117 hp at that rpm to even begin to produce the same thrust.  Props begin to lose efficiency as they get smaller and spin faster so even the same torgue probably would not produce the same thrust.  

The next question is whether the VW could even produce 117 hp at 3000 rpm.   All engines produce nearly the same volumetric efficiency. This doesn't vary by more than a few percent.  That means that you can't get more hp unless you move more air.  If a 200 cubic inch engine turns 2550 rpm then it is moving 200 *2550 cu in of air or 510,000 cubic inches per minute.  The biggest Volkswagen barrels are only 2100 CC which is equal to 128 cubic inches.  That means that turning 3000 rpm the only moves 384,000 cubic inches per minute of air.  384,000/510,000 =.7529  which means , unless someone can change physical law, the VW engine can only put out that portion of the Continental's 100 hp or in other words a 2100 vw should put out 75 hp and a pitiful 131 ft lbs of torque.  Maybe some people can do magic but numbers are pretty truthful.  There are some advantages to better ignitions and carburation as well as compression ratios but without turbocharging, using nitrous oxide, or something extreme they are still a long way off.   

The Subaru I had was 109 cu inches turning 5000 rpm.  The air volume was 545,000 and they ONLY claimed 100 hp at that.  The prop rpm was through a reduction unit so it was only 2400 rpm with a resulting torque of 218 ft lbs.  That is a combination that works fine.