Hey, I'm a big fan of portabilizing and modifying. I'm currently in the process of making another N64mp.
It is easy to get screens. The best screen to use on most systems is actually a PS1 screen(because they are easy to adapt to most systems), which is usually $10-$15 on eBay. What system does your son want to mod? Most PCs (Portable Crusaders) use either Radio Shack battery packs or Ni-MH(1.2V each). For battery ideas, check **broken link removed**. This is actually a really good project for friends and families to work on together, because if one person has problems, the other can help. I will tell you right now, if he is going to use a Nintendo system(like N64 or GameCube), you will probably need to either use the melted pen technique(search the BenHeck forums), or buy a Nintendo screwdriver(inverted torque wrench from **broken link removed** or eBay).
Here are 2 great sites for portabilizing and modifying almost any system ever made:
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and
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