Although it seems like most people who "work" on mopeds have never heard of it, there is this great tool called a compression tester! Really, its crazy that anyone would actually make a tool that is more acurate than watching for soap bubbles on the side of you motor, or just listening, cause those seem like pretty solid ways to make a definitive diagnosis, but you can get a thing that will actaully tell you exactly what the compression in you motor is. There are cheapo options that will probably be fine for what you need to do, and there are much more expensive options, which are probably not necessary for you. Go to the local autoparts store like autozone, and tell the guy at the counter you want a cheap compression tester. Then take it home and use it, and see if your compression is what the manual says it should be. If its not, tear off the head, and see whats going on inside. Scored walls, worn rings, bad headgasket?