Fire is of great importance in many survival situations, and the Spark-Lite is one of the better things to carry in order to allow you to make fire. It is small, light, easy to use and reliable, and as a bonus, can be used with one hand. In the day (30 years ago), they were made of brass, which was solid and durable, but heavy and probably too pricy for today. The nicest feature was that the bottom was a threaded knob, so the flint could be replaced.
The latest ones are plastic, so are lighter in weight, but they are all one piece, making the flint not replaceable. This is not really a problem for most kits, which are carried much and used little. And the flint that is in there is about twice the length of most flints. But if you really want this capability, you can modify the plastic Spark-Lite to allow changing the flint.
This requires you to cut into your Spark-Lite, which means there is a real possibility that you could destroy it. For that reason, it is strongly suggested that you get a spare one to make this modification to.
The way the plastic ones are (or at least recently were) made, is the body is cast with a threaded hole at the end, the spring and flint are installed, and a threaded tube or rod is screwed in and fastened, probably with some combination of glue and/or heat, then trimmed to the end of the body. So, if you cut down to, but not into the threads, below where the rod is fastened, you can screw off the bottom part of the body with the rod attached, allowing access to the spring and flint.
From the end opposite from the end where the spark wheel is mounted, measure 3/16″ and mark the body all the way around (all four sides). Using a razor saw (very thin; the blade I used was from an X-acto knife set), cut 3/32″ into the body along the lines, all the way around. Go slowly, since you only want to cut up to, not into the threaded rod. You will need to saw a bit deeper around the corners, since the threaded rod is round and the body is square. As you get close to the threaded rod, keep trying to unscrew the bottom part (the part below where you are cutting). Once it moves, you can unscrew it all the way. Be careful, the spring will try to jump out.
As an aside, it is possible to cram 15 Tinder-Quik into the Spark-Lite case. Two by two from bottom to top, for seven layers, and one flattened and laid on top. If you are carrying spare flints (which is kind of the point of this exercise), put them under the Tinder-Quik, not under the Spark-Lite, as it will not only keep the Spark-Lite from seating completely into the case, but the flints can easily fall out of the case.