No, it's not necessary to have an air filter over the carb. You can put one on if you want, but I've never seen a guy with one at our club and have never personally had a problem without one; however, a fuel filter is a good idea. You could install one on the line going to the carb (between tank and carb). When you fill up, take the line off the fuel filter (NOT the carb) to fill.

That's a nice airplane. Want to keep it nice? Find an instructor at a local club. These R/C airplanes are MUCH more difficult to fly than most people realize. If you still aren't sold on joining a club take a look at my latest blog entry.

By the way, the carb looks off center... Is it nice and tight? If not, push on the carb with your thumb as you tighten to get a good tight fit. In fact, it looks like you're missing a screw on the carb. Before you fly make sure you get those clevis keepers (the small bit on fuel line at the end of the control rods) on the clevis. Another good idea is to point the muffler down away from the wing. This way the crud can get out of the engine easier and your airplane will be cleaner. To aim the muffler loosen the bolt at the back and remove. Then loosen the screw in the front. That screw actually goes all the way from the front to the back. Those threads you took the bolt out from are part of that screw. Twist the back of the muffler to aim it, then re-tighten the screw in front and put the bolt back on.