That XR1200 has a great looking motor which even appears to have twin carbs. But I would not fit one into a featherbed frame. The problem with most Norvins is the weight distribution - 50% front and back is quite common. With a Mk3 Seeley frame, you can probably get the motor further forward. It is easy to build a motorcycle which can scare you shitless. Building one which handles well is more difficult. I think S1 Buells looked promising, but the rest were extremely ugly.
I think Americans have a need to be different, so a British frame is out of the question.