I downgraded my AK2i (HWID 81) using this original firmware: https://filetrip.net/nds-downloads/flashcart-files/download-acekard-original-bootloader-1-0-f33562.html
In spite of the warning that "This won't work even on a CFW'd 3DS", it seems to boot both the kernel and games fine on my N3DSXL...