GBA games on N3DS aren't emulated , they are running natively on the hardware cause the hardware supports it. Open AGB Firm and the Virtual Console Injects use this hardware support to provide native playback.
Now, you could emulate it with the N3DS port of mGBA but that's like the worst way...
I was able to load a mod, it just crashes when starting a cup though
also for some reason, the modder just left the png files of the character icons and left the user deal with it, which is dumb asf
UPDATE: got it to load but crashes when loading into a track, my big mistake was using CTRL + F to Find "MARIO KART 8"on a title database and not pay attention to the sections, the id on the folder was the DLC ID. im glad it loads it but it doesn't really matter when the mod makes the Wii U Crash
I may try the new ram site, and download more RAM to my Switch. Not sure if ddr3 is the right ram
for it tho. Edit- no it uses floppy Ram, just like @AncientBoi