Beta RR 50 engine failure for no reason? (Am6)?

I have a 2015 Beta RR 50. The first time the engine failed, the previous owner drove it without oil. They installed a set of 70mm oil which did about 7,000 kilometers. Then one day it just died on my way home (right when I stopped). I replaced and cleaned almost everything until I opened up the cylinder and discovered that small pieces of metal had broken off from the piston and cylinder head. However, there were no loose parts in the cylinder; they had all become stuck to the cylinder head or piston. I then blew out the engine and cleaned it and then I fitted a 75mm oil filter (I have a 21mm carburetor, a larger air filter and an approved exhaust for the cylinder). When I fitted the new cylinder, it ran perfectly for the first 25km. I could always start it perfectly with the first kick, even by hand. Then all of a sudden I lost power and then it died again about a kilometer later when I stopped. I've now opened the new cylinder again and am having the exact same problem, even though I'm pretty sure there were no foreign objects left in the engine (if there ever were any). I've attached three pictures. The picture of the piston and the darker cylinder head is the old cylinder, and the lighter, clean one is the new one.