In response to your question,
#1 might not be able to be done since maybe the standoffs might be arranged different on the G3 motherboard versus the G4. As for the rear ports, either remove the back panel with the 2 screws in each corner holding it on, or do what others have done. Used a drill to drill a hole to where the one sound ports lies down on the motherboard.
As for option #2, that might be easier. Since the outside case plastics are held on by 4 allen type screws on each of the corners. Same with the handles as they have an allen type screw on each side holding them to the metal case. The only difficult part would be the sides since they are clipped to the center of the case with a couple of plastic tabs that have to be pressed together in order to release the plastic from the metal. Or do what i do. Gently apply force on one side of the plastic while trying to pry it up and being careful not to break the plastics. It should pop off and then you can transfer it to the new case then use the allen screws you took off to reattach it to the case.

I hope this helps you out.
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