NO PUNCH! pipedr has the right idea. The more agrresive part is bad you can crack the head. I restore antique Brittish bikes myself. If you have a big enough pot you dont care about take the head off with the exhaust attached and put it in the pot filled with motor oil and heat on a stove type burner. heating it in the oil slowly will prevent warpage (also works well for fitting pistons if you know how)as it expands the oil works into the area around the pipe as well as the aluminum will expand before the steel. Should make it easyer to pop loose. Just remember patience and not too aggressive.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!