I had a Testors McCoy .35 Red Head turn from red to black in short order. Engine was run rich, used a 10x6 prop, so I wasn't abusing or overheating it. Appears the previous owner repainted the head in a non-heat resistant paint. Could it be that the darkening of your Torpedo's head was the result of the same? Once you get the paint removed, you can repaint it with a good quality heat resistant paint like some auto engine paints.