Thats wierd. At least for the 3 grain 54mm size, the grain length of each grain is the same for Brand X and Loki. So concievably, the only reason it would not fit is the length of the liner tube is different. If loki uses longer liner tubes, then it could be made to fit in a Brand X motor by cutting the liner tube.
However, doing any of this or even flying it in the case without modification would be an EX motor and need to be launched at a TRA Research Launch by a L2 flyer.
It wasn't the length, as I remember it didn't fit because the liner was way too thick. Fit perfectly in the Loki 54-1200 but not the corresponding BrandX.
Then it comes down to AL tubing supplier. The industry tolerances aren't as tight as it needs to be for rocketry usage. The only way to ensure a proper fit would be to have a custom mill run like AMW did with their hardware & that takes $$$$$$$. Then you pretty much eat up you margins & have to charge just as much as AMW if not more for the hardware. It's EX and your not going to make a dime on the reloads for any hardware.
It's easier to sand down the liner or try another than it is to hone out you Al casing...