bare minimum you need boot.firm if you were using Boot9Strap (FW 11.6/ Luma 8 or luma 9) or 11.5 n older you might of been using A9LH, and that case w/ an oldre luma build you'd need arm9loaderhax.bin on the SD Root. now if you Were on A9LH, i'd suggest following the guide to upgrade to B9S, (as no more updates to luma if you stayed on A9LH, and its a lil more streamlined, outside that, theres no reason if the games that need a newer FW dont tickle yer fancy to give you a reason to update.)
that and depending what ya had installed, you deffo need if you still have it, your old "Nintendo 3DS" folder to regain your saves the "3DS" folder which retained things for homebrew, freeshop and other stuff. but outside those the bare essential to be usable again to boot your modded 3DS depending how old a mod your using (Menuhax, A9LH, B9S....) you only need boot.firm for B9S and arm9loaderhax.bin for A9LH from a luma build. or other supported CFW of your choosing.