You can have 1/4 x 3/8 spar notches as shown on the plan for hard balsa spars, or 3/16 x 1/4 for optional hardwood spars.
I built one of these a few years ago, powered by an OS 15 FP. It flew even better than I expected. It’s tempting to think that there will be extra trouble when you try to fly a seaplane, but this one flies like any well designed RC plane. It’s smooth and predictable, it lands well, and it even does aerobatic stunts. The Drake II is a very simple build, so if you have ever considered building a seaplane there’s no reason not to jump in and do it.
The engine pylon is the hardest part, but it’s already cut out for you. The pylon is designed for a .15 glow engine, but it’s easy enough to saw off the engine bearers and add a flat plate for mounting an electric motor if you’re one of those people.
The plan shows a nacelle built from small bits of balsa and a tail cone made from a little balsa block. The kit does not include any of these parts because various builders will build the nacelle to match their own battery or fuel tank size, so there’s not much point in me cutting little rectangles that you’re just going to replace anyway.
The parts are cut pretty much according to the plan. If you want to build it with ailerons, just trim 1/4″ off the tail of each rib and put a stick on the trailing edge, and use a piece of tapered stock as an aileron.