ORIGINAL: loughbd
The Cox glow plugs are readily available and a replacement head that uses a standard glow plug has been around for years. You were lucky. trying to get the plug the exact depth for the correct comprssion is about impossible. We have been trying the drill out the old head and tapping for a standard plug for at least 40 years.
You don't just tap some threads into it. The thread is a 1/4x32 and difficult to find. Higley sells one.
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Several vendors used to sell just such converted Cox glow heads quite regularly. The downside was that the engine did not produce quite as much power using the modified head. Sport flyers of these engines were not concerned with the slight power loss and enjoyed the cost savings.