After scouring the webz for some decent BN-2 drawings, I came up empty. I did, however, find a reasonable Trislander drawing, complete with [some] dimensions. Unfortunately, it's only a profile view, but it's a start. I imported the drawing into Autocad, and found that the X and Y scales on the drawing were different. After a bit of faffing about, I scaled it properly, and started tracing it. Once I'd done that, I copied what I'd done, made an edumacated guess or two, and began to convert it back to an Islander.
Here's a photo of what I've done so far, with the Airfix fuselage underneath...
....and on top..
Well, my guesses were pretty much spot on. There are three areas that still need work - the nose, the windscreen, and the end of the fuselage. You may notice my drawing is slightly longer than the kit piece, and I'm not sure whether to ignore it and assume the drawing is correct, or tweak it to fit...