Use foam rubber to secure the tank. Make sure that the tank is supported by the foam rubber on all sides, and won't move around.

Most fliers don't use an air filter. Unless you're flying off of dusty dirt, it's not an issue. A gasket between the muffler and engine isn't necessary either. Some locktite on the bolts that secure the muffler isn't a bad idea, though.

Have you found an instructor yet? If you've never used glow engines, I'd strongly recommend that you find someone who's run them to help before you go about breaking it in. If you intent to break it in in the plane, you really need the radio gear installed and set up before you proceed.