You will need a Metalworking Lathe for this
Parts are not always available for old model engines, getting parts for the early discontinued Laser engines can be a problem
I made a new piston ring for a very old "LASER" engine owned by a friend. The previous owner had broken a quarter of an inch section off the original ring, the engine still ran, but not very well & parts were unavailble for it as it is a very old engine! I am not a trained engineer so apologies in advance to the purists! Here's how I did it: Remove the cylinder head assembly, so as to get to the piston & liner. Using vernier calipers, carefully measure the depth of the groove on the piston (remove the broken ring first) using the same vernier calipers measure the internal bore of the liner. Obtain a piece of high quality cast iron, hold in a 3 jaw chuck on a lathe, even a very small one will do!
Depending on your equipment, machine the cast iron slowly - without proper carbide tipped cutting tools, if you go too fast you will soon blunt high speed steel & the tool will need regrinding frequently. Just go easy on the iron, later on you will smash it to bits if you are too heavy handed! Centre drill the cast iron bar, slowly drill in a couple of inches or so, open out & finish bore the internal diameter to almost match the piston's groove diameter - bore it slightly bigger by to allow for the closure of the gap in the ring later. Turn the outer diameter to approximately 50 thou above the diameter of the bore of the cylinder liner. You should now have a cast iron tube about an 2 inches long. Part this off & remove the rest of the cast iron bar stock from the chuck. Using a very fine slitting saw, carefully cut a slit all the way down one side of the tube - do not clamp the tube in a vice!. Clean up the slit gap with some "wet or dry" sandpaper or fine emery cloth.
Next, Put a piece of mild steel bar in the 3 jaw chuck & turn it down a "couple of thou" larger that the piston's groove depth for a couple of inches. Slide your cast iron tube over this mandrel & clam it tightly to the mandrel with a couple of "jubilee" clips. Make sure that the slit in the side is firmly closed by the pressure of the "jubilee" clips. Finish machine the outside diameter of the ring to match the liner's bore Very carefully, part off the ring to match the groove in the piston, err on the "very slightly too thick" side!
DO NOT remove the mandrel from the chuck at this stage You now nearly have a piston ring, but not yet! Using a "whetstone" or wet or dry sandpaper with a little oil on it, clean up both sides of the ring to a good finish. Test the gap in the piston s you go, but you do not need to fit the ring, just slide it into the groove to see how it's doing. When you are happy with the fit, (it must not be too tight) very carefully fit the ring to the piston after first removing all traces of gritty oil from the sandpaper process. Be careful here, the ring will allow expansion just enough to get it over the crown of the piston. If you break it, play J.S. Bach's "Air on a G string", smoke a cigar, & repeat the above process. This is why I recommended to machine a 2 inch length of piston ring material & not to remove the mandrel from the chuck. Once the mandrel leaves the chuck it is time consuming to get it to run accurately. If you have a Collett system it's better! Once you successfully fit the new ring, carefully re-assemble the engine using copious amounts of oil. When first running the engine, treat it as you would a new one, it needs a little running in. There you have it then, an easily made new piston ring & an engine that works again.
The old Laser 75 runs as it should now.