I'm actually rather a fan of 'American engines in British frames' bikes, my original 45-powered Ariel was quite an eye-opener as to what a bike usually regarded as a heavy old slug could be like with a bit less weight, and better carb and transmission. I've seen pics of a knucklehead engine in a Norton frame ( anyone got any pics of this? It was in one of the 'grey import Harley' mags that were about in the late 80s ) and a Norton-framed Sportster.
another half-remembered bike I'd lke to see some pics of is the KR-framed Sportster-engined dirt bike Harley built for desert racing in the late 1960s, I remember seeing it is Cycle ( I think ) magazine. From recollection, it was tried in the Baja and had a history of breaking itself and not finishing, before being quietly dropped after a couple of years