$21 is a steal with how prices have been trending over the past two years.
How much money are you trying to put into it? That really determines what you're looking to buy. If you want to use the original screen you need to VERY CAREFULLY remove the polarizing film from the front and back of the LCD, probably, as it's burnt on one side. This process
fucking sucks, I've tried it, and ruined one screen in the process. Replacement original GBP screens are difficult to come by, and they're all pulled from consoles at this point, no new-old-stock or aftermarket alternatives.
If that's the route you want to go, which I really, really can't recommend in 2021, you're looking at this for the screen.
2x of
https://handheldlegend.com/collections/game-boy-pocket-mgb/products/polarization-film (I believe you need 2)
1x of
https://handheldlegend.com/collecti...ucts/game-boy-backlight-v3?variant=4515813315 (multiple color options available)
Optional, 1x of
https://handheldlegend.com/collecti...ket-bivert-module-mini?variant=16058480885803. If you're going through the effort of doing that kind of screen upgrade the bivert mod is easy enough to tack on top.
You want to ball out on that thing?
IPS conversion kit -
https://handheldlegend.com/collecti...cket-ips-lcd-backlight?variant=31741225238662 (STRONGLY RECOMMENDED)
(note, do not use a bivert mod with this, unnecessary and will invert all of your colors)
Tempered glass lens, multiple colors/designs -
https://handheldlegend.com/collecti...oy-pocket-glass-screen?variant=34823816937606
Audio amp, unnecessary, does make it considerably louder but also disappointingly makes it mono from the single speaker instead of stereo.
https://handheldlegend.com/collecti...h-speaker-for-game-boy?variant=31485692248198
New power regulator
https://handheldlegend.com/collecti...t-cleanpower-regulator?variant=39408079143046
New power switch if yours is damages
https://handheldlegend.com/collecti...universal-power-switch?variant=39431120683142
Recap kit in case you feel it's needed
https://handheldlegend.com/collecti...capcitor-kit-console-5?variant=39248179495046
Rechargable battery and wireless charging
https://handheldlegend.com/collecti...ck-for-game-boy-pocket?variant=39436346425478
New speaker
https://handheldlegend.com/collecti...ducts/game-boy-speaker?variant=31485741301894
The site also sells replacement shells, buttons, membranes, battery doors, rear stickers, battery contacts, and tons of other bits and bobs to make your restoration as fresh as possible
https://handheldlegend.com/collections/game-boy-pocket-mgb
Not shilling they're just one of my favorite sites to order from for restorations like this. There's plenty of other options out there for some of this stuff like Retrosix, Castlemania games, console5, and a bunch other I'm forgetting.
Edit - the "ball out" build is what I'm doing to both of my Pockets that I have. One may get a fresh case, one will likely stay original. And I think I'm doing the IPS conversion to one and the slightly cheaper TFT conversion to the other.
Edit 2 - Oh and for games, if you're looking for a flash cart the EZFlash Jr is probably your most economical option. I have one and it works quite well.
https://www.ezflash.cn/product/ezflash-junior/ https://www.ezflash.cn/dealers/ If you're looking to save a few bucks you can buy them on AliExpress but they are likely clones and if you try to update the firmware on them you'll likely brick it. There is an Everdrive for the GC/GBC but they're so unnecessarily expensive for, in my opinion, little to no benefit over the EZFlash.
Edit 3 - Game suggestions in general, from personal preference - Gen 1 and Gen 2 Pokemon (Red, Blue, Yellow, Gold, Silver (Crystal is GBC only)), Pokemon Trading Card Game (hasn't aged particularly well but was fun in the 90s lol), Super Mario Land 1, 2, 3 (3 is Wario Land 1), the Game & Watch collections are great for quick minigames. Donkey Kong ('94, not Donkey Kong Land) for the Gameboy is a PHENOMENAL game, can't recommend it enough.