Let's get started on these case modifications!
It's clear from looking at the case, that in order to fit everything in there, we're going to need to make some cuts. It's time to let your cutting tool do the talking. It's important to note that, on the rear case the Game Gear has plastic standoffs to support the buttons. Don't cut these standoffs or otherwise ruin their structural integrity, or your button performance will suffer. It's also wise to leave the main case screw standoffs intact for the same reason.
You can see where they line up here! Mark them so that you know where they are when you're hacking up the case.
Below is a diagram indicating the spots to explicitly avoid when cutting your case. Be as selective as you can when you're removing pieces. Don't slash and burn it (even though, that's more fun). Run through your space planning a couple times before you mark the case for your cuts.
It should look something like this when you're done. By far, the largest items to place in the rear case are the LiPo battery and the Raspberry Pi. You may want to slightly alter your cuts if you're using the Raspberry Pi A+, as you need far less room compared to the others.
If you care to be more agressive in cutting the right battery compartment, you could certainly shift the Pi over to the right by a couple centimeters. Just keep in mind the button supports and main case standoffs!
Next up is the front panel. The Game Gear originally came with a 3.2 inch diagonal display, which is just shy of the 3.5 inch diagonal display we've chosen as ours. You can remove the front plastic bezel, by pressing on it with your fingers from inside the case. It should slowly pop forward. Slightly heating it up with a hairdryer (Not too hot!!) will make it a bit easier, but is not required. There's also this small rubber display gasket, which can be peeled back and removed.
Cut slightly below this line here, about 1mm down the imprint. Do your best to keep the cut even. File down any rough parts when you're finished. The bezel will hide imperfections for the most part (If you even have any, you awesome maker you).
While you're cutting up the front case, you should delete the standoffs that surround the LCD bezel. They will interfere when we attach the display to the front case. The pictured cut is slightly too large, so aim for a bit smaller than this with yours.