There is a tool somewhere that tells you the hex values for card manufacturer and the unique id of a gamecard dump that was made with gateways launcher (doesn't need gateway). Take those values and put them in the right places in template.txt (take a look at
https://github.com/lukas2511/sky3ds.py/wiki/Disk-Layout#bytes-from-templatetxt for which value means which), i think you'll mainly edit the first and the fifth line for the game, not sure what is important, probably mostly uniqueid and cart id, but maybe also eeprom id. For my try i just replaced everything that I could get from my original gamecard, and it worked.
Some people probably posted on here how to get "online headers" or something running, that is probably done the same way, so maybe read one of those tutorials, dunno.
edit: you can also get those values directly from a 3dz file with a hexeditor, i think somewhere around 0x1200 or something like that, it should be easy to spot as at least the unique id is surrounded by lots of 0xff