There used to be a quarterly magazine published in the UK called International Movie Maker. It was produced by Roy Salmons who had a shop called Photoworld in Craig y Don, Llandudno. I subscribed for a few years as it contained lots of good info about small gauge cine. I'm sure there was an article at one stage about re-loading Super8 cartridges. From memory, however, it was not that easy. I'm guessing you have some Single 8 stock you want to use, but the cameras have been unreliable? I've never used Single 8, or had a hold of one of the cameras, but they were mentioned regularly in that magazine. Users seemed to be very keen on them. I have bought, and used, a number of Super 8 cameras over the years and they have all worked. The most reliable has been a Canon 514XL which has the advantage of running on two AA batteries without a separate meter battery. Some of the older models, like the Bolex 150 series don't always give accurate exposures, but more recent cameras have all been ok. I know the claimed advantage of Single 8 is the use of a proper gate in the camera to keep the film flat, but I've had some fairly good results with Super 8. I suppose it depends on what you are filming. I would think that re-spooling Single to Super would be risky, and a last resort. If you have a load of Single 8 film and want to move on to Super 8, you might be safer selling it.
Alex.