Last Weekend at the Harley Davidson dealership in Maryville Tennessee. They were having Smoky Mountain Bike Week. These are mostly the display bikes in the dealership.
Allen Millyard is a legend. He has a somewhat irrgular youtube channel and he'll do things like casually add a couple of extra cylinders to an engine, with as little trouble as most people have to change a spark plug. He's built all kinds of stuff. A V twin with aircraft barrels and pistons and a couple of V12 Kawasaki Z2300s, by adding another cylinder bank to a straight six. I think the Viper bikes' greatest success is that it isn't totally hideous, as these shed built things tend to end being. It looks like something a factory could have built.
I was about to say, I love Millyard's work, but car-engined bikes are extremely hard to pull off. I think your assessment of the Viper bike is about right. Even car-engined trikes are easy to get horribly wrong. But that the car-engined bike is a nut which has not yet been cracked does not mean that it can't be cracked. For a start, putting the radiator almost anywhere except behind the front wheel would be worth exploring.