First of all flip it over and there are six screws to undo:
When you have done that, VERY carefully hold it together and flip over onto its bottom, then slowly pry the top away:
You can now also remove the back fins:
It will now look like this:
You can also remove the guns as they are only held in place by the top and bottom halves.
Here is the motor:
There is the cog and the small piece of plastic under it that together make the noise (the cog may need to be loosened and moved by hand):
The problem with mine was that the two copper contacts werent touching when the button was pressed:
I had to bend the copper slightly at the top so that the two contacts would touch.
So, put the back fins back into place, then carefully balance the guns in to place and put the top back on. BE CAREFUL not to disturb the landing skids!
Then hold it in place flip back over and CAREFULLY screw them back into place, you may need to hold the ship together in some places when screwing, do NOT overscrew or the plastic may split!
Hope that this helps.
Photos and text courtesy of Chris Chilton.