Tower Hobbies has a Fox needle valve for $12, a Tower version and a Perry version for a few more dollars. The K&B needle valve should be available from your local hobby shop and many other online hobby shops. Any of them should be okay.

The gasket you want to verify is between the head (where the glow plug screws in) and the cylinder liner beneath it. It is a thin brass gasket. If it is missing, you can order one for a couple of dollars. When you replace the head, tighten the screws each a little at a time, working back and forth across the bolt pattern, not sequentially around the bolt pattern. I don't think you want to use RTV between the head and cylinder liner. RTV is fine between the muffler and engine, although I usually don't bother sealing this joint. Any minute leakage here has no effect on engine performance.