I can't open the battery lid because the screw is broken

This fix takes a very experienced technician to do it safely. That having been said, the screw length of the GBA Sp are all the same depth (length of the screw). It is important to not exceed this depth when very slowly and carefully drilling the screw out. Yep, just like any other stripped screw, it has to be drilled out with a bit just slightly larger in diameter that the screw. You will then have no choice but to replace the screw “nut” holder for the battery cover and of course get a new screw. I realize this question was posted in 2017 and it’s being answered in 2024. It is being answered for anyone else who has this question. I do not respond to additional questions like where do I get “? Part or tool”. If you are actually asking this question of where do I, you have no business attempting this repair. Just take it to a shop willing to work on it.