there is no alpha mask channel. because the original image was indexed, the alpha channel has effectively been destroyed.
(more like, it has been merged. it is now a part of the RGB channels)
if you wish to edit whatever on it, you will just have to delete what you dont want, and redraw it.
only way to mess with the alpha on the image is to take it back to its indexed form and change the palette alpha velue entries by hand.
ofcourse, as stated earlier already, this requires specialized software, since photoshop and paintshop pro etc. cant really handle it that well,
which is undestandable since indexed png's are not really common at all in everyday usage.
imagestudio can tamper with the palette alpha (see pic), but even so, you would have either restrict your edits to the existing palette, or create new palette entries for new color/alpha combinations you wish to use. It's really easier to just redraw the whole thing.