I had pondered that point in another thread- my thinking is, keep the shrouding on, since the engine and finning were designed for it. If you look at a typical non fan-cooled motor, they have pretty healthy sized fins compared to a fan-cooled motor. Also, from the Olde Days, when people were yanking their belt-driven fans off of their cars for those precious-few extra horses, the recommendation was "anything over 35mph doesn't use the radiator fan". My bike doesn't see much over 25mph, typically.
Now time for some REAL NERDISM... I found, a few years ago, an engineering paper written by the developers of the Kool-Bore engines for Briggs & Stratton. It was absolutely an amazing read. In it, they spoke of stress-testing the prototypes by blocking fins, removing fins, removing fins from the flywheel, etc, to see how the design would fare... the astounding part that they found, was that the one piece aluminum block did a great job of conducting heat from the hot-spots, down to the crankcase and whatever the motor was mounted to. And it makes sense, in the case of a typical B/S Flathead... they generate roughly 1hp for every 3 cubic inches, so they're not a stressed design, stock. NOW- can I find that paper or webpage ANYWHERE on the "net" now? Nope. If I ever DO find it again....
In the case of the HS 142F and 144F motors, I'd think that they NEED the cooling in place, because they are spinning much faster and putting out almost double the HP per cubic inch of a B/S flathead (stock).