DB, as usual, has some good advice there. If you really want to do a nice fabric cover and have the patience for it, Sig Koverall and traditional dope finish gives a very nice, durable and light cover. I'ts not really hard to do, but is more time consuming than a prefinished cover, because you have to apply the glue to the edges of the airframe, then cut and apply the fabric, then shrink it, then dope it to seal the weave, then paint as desired, but the edges of the cover just disappear, and it NEVER wrinkles.

As far as engine choices go, I don't think you can do any better than Saito, but the OS is also good, and a .91 will be all the power you need and then some. Do you know the projected weight of the plane? My 1/5 scale L-4 (84.5 inch wing) weighs over 9-1/2 pounds, and a .65 Saito flies it, no problems. It takes off at 1/2 throttle and cruises at 1/4-1/3. I mounted mine inverted, and it is almost completely hidden in the cowl, then I built a scale dummy engine. I have an on-board glow, which I recommend for inverted applications if for no other reason than to make it easy to start, since the glow plug will be almost impossible to get to. It does provide a good measure of reliability for low throttle operation, which you will do a bit of since you will have such an excess of power. Your plane is a bit larger than 1/5; a 14 inch prop is scale for 1/5, that's close enough for you and the .91 will turn it great. I use a 14-6 MA K series. As Db said, the 3004 servos are fine, I used 3001's in mine, almost the same, have the same specs for torque etc, and are plenty

Learn to fly that trainer well, while you're building the Cub--you're still gonna have another learning curve when you begin to fly it. A trainer is great, but let's you get away with a good bit of "hands off" flying. The Cub wants your attention most of the time, and has handling quirks that, while they aren't bad, require getting used to. Taking off will teach you how important that rudder is, and making coordinated turns will also show you how to fly with both sticks; I hope you enjoy it as much as I do, it's my favorite plane to fly You're taking much the same course I did when I started flying. I built my trainer from a kit, and then started building my Grasshopper while I was learning to fly with the Kadet. My L-4 is a Sig kit, I modified it myself as I went, using data I got off the 'net. Take your time building the Cub, however you decide to do it. You'll do a much better job, it will be a better plane, and it'll be worth the wait.

One last question. What radio are you gonna use? If you've got a computer Tx, there are mixes and exponential setups that will help make the Cub easier to get the hang of, such as mixing rudder with the ailerons to help with coordinated turns, putting expo in the rudder will reduce its sensitivity and help reduce the tendency to overcontrol the tail on takeoff, and differential in the ailerons to help reduce adverse yaw (along with the rudder mix) in turns.