I've repaired about 20 different Gameboys. The community for Gameboy repair and mods is huge. I suggest watching a few of macho nacho videos on YouTube to work up the nerves to open it. They are so easy to fix, and there is a modern replacement for almost anything you can break. Also take a look at the ifixit guides. https://www.ifixit.com/Device/Game_Boy

https://www.retromodding.com/products/game-boy-battery-housing-contacts

fix if yourself, itll be a good experience.

It would help if you tell us where you are located geographically and what specifically is wrong with it that needs repairing.

This guide might be helpful: ifixit game boy battery corrosion

1up restorations modded my NES for RGB - highly recommend them. https://1uprestorations.com/