ndsi-savedumper VS TWLSaveTool

First, follow dsi.cfw.guide to get homebrew setup on your DSi (you must install Unlaunch or it won't work). Then download DSi save manager and use that to dump your save file. Then you can move it to your PC via the SD Card

For DS (Requires flash cart)For DSi (requires Modding)For 3DS (Requires Modding)

Restore that edited save file to your game using savedumper.

Just to add to this, there's also ndsi-savedumper. It's no longer in active development, but you can get it here: https://github.com/edo9300/ndsi-savedumper

You can use a savedumper (https://github.com/edo9300/ndsi-savedumper) for dumping and injecting saves from and to legit cartridges. If you're playing a gba game on the DSi, you will have to install GBARunner2 (https://github.com/Gericom/GBARunner2) in order to run gba .rom files. I'd try placing your .sav file next to your .rom file and naming them like this: "{filename}.rom" and "{filename}.sav", where the {filename} part is identical.

ndsi-savedumper

I assume based on your cfw comments you already backup your ds/3ds games, but for anyone googling or searching this reddit in the future... TWLSaveTool can backup your DS games. 3ds has Checkpoint or JKSM. You need homebrew or a cfw 3ds for these.

You could try TWLSaveTool which is an NDS save dumper/restorer for 3DS, but it could be that whatever file structure the emulator used isn’t compatible with the OG cartridge.

Since it's a DS-era game, you shouldn't be using Checkpoint, btw. Have you tried using TWLSaveTool?

If you want to extract the save from the cartridge you can use TWLSaveTool

TWLSaveTool

Rudolph’s GBA backup Tool / TWLSaveTool