The-Winged-One, true confessions time!
Once, and only once, I broke my own rules. I'd set my heart on taking a B25 Mitchell (range say 1,500nms.) RW north-south. Only thing was, there are virtually no airfields in Antarctica - and those that there are were too far apart. So on that leg, and that leg only, I cheated and used 'Aircraft/Fuel' to stick in another 500 gallons.
I'd recommend that you do the same thing on the Iceland/Scotland bit. Not worth spoiling the whole 'adventure' for just one leg - as you imply, what you'd do in real life is have an auxiliary tank fitted.
I think that, if your AG14 really has got a 1,000-mile range, you should be able to manage the rest of the trip - though it will take a helluva long time, and you'll find yourself landing in all sorts of odd places

. And getting over Greenland (9,000 feet) and through the Rockies (12,000 feet minimum, and even then you need to work out your route carefully) will be pretty challenging.
In any case, I really would try a couple of experimental flights first, and see what sort of extreme range you can squeeze out of the aeroplane. Essential before you can really plan the trip.