I can't help you with the legal requirements for the flight because it's probably all changed since I did it 10+ years ago but, if you have the opportunity to make the flight and can clear up the licence aspect, go for it - you may never have the opportunity again.
It's a wonderful trip and, although the time over the Atlantic must become boring for the regular ferry pilots, I'd be very surprised if you're bored the first time. North America from the air? Greenland, with its mountains, fiords and icecap? Not what I call boring!
I can see Doghouse's point about leaving the owner behind and taking a more experienced pilot, but isn't the danger that the owner will go with the more experienced pilot and you'll be the one left behind.
I did it with an experienced pilot, but neither of us had crossed the Atlantic before. Read everything you can, speak to an experienced ferry pilot, and plan, plan, plan. By the time you set off, you should know the trip in minute detail in your mind. We didn't have the luxury of GPS 10 years ago but if I did it again I'd take two, and be ready to carry on if they both went u/s! Greenland doesn't have many airports, the diversions are a few hundred miles away and, if you're routing via Narssarssuaq (Southern tip of Greenland) a few degrees adrift and you could easily miss an entire country!
Try to get hold of a book called 'Ocean Flying - A Pilot's Guide' by Louise Sacchi, published by McGraw-Hill ISBN 0-07-054405-0. It's pre-GPS, but a mine of valuable information about every aspect of flying the North Atlantic. I don't know if it's still in print. If you can't find a copy you can borrow mine, on your solemn undertaking to return it. It's of great sentimental value.
Tudor Owen