We could tell you more if you could tell us how it doesn't run. You may need a new glow head. Cox engines start and run best if the glow plug really lights up. There is a copper gasket between the head and the cylinder; If not, get one and put it in. Check your needle valve. Sometimes the tip of the needle will break off from vibration. If so you need a new needle and need to get the broken tip out. Another possibility is that the reed valve which lets fuel and air into the back of the engine is stuck or has a piece of dirt in it. Check and see if the back plate is on tight. It may have vibrated loose enough to cause a leak in the crankcase.

All of the above are easily fixed. The connecting rod joins the piston in a ball and socket joint. This joint will loosen with time, and left alone long enough the rod will beat through the top of the piston (speaking from experience here). I imagine your rod needs to be reset. Carefully turn the propeller until the piston is at the top of the cylinder. Now gently wriggle the prop back and forth a little. If you can wiggle it much with the piston staying in place, the piston needs resetting. Cox makes a tool for this, but if you can find someone who knows about engines to help you, best do so. It is easy to ruin the piston trying to reset it (experience again).

Hope this helps.

Jim