I used twin magnum 25 engines on my seamaster 40 built from plans. You don't need to scale it up any. The 25's will be just right for the 40 size. One engine will still fly it in the air, I would not try and take off on one though. I built the wing flat all in one piece and used hemlock for the main spars instead of balsa. The plans I used were free on line and are the lightened version of Ken Willards plans. I can't remember what web site I found them on but if you search around you should be able to find them. I have a write up of my build here on rcuniverse somewhere. Here is a picture of my twin. It handles really well in the water and flys great.