cheapeto caught that fact that this is probably not the typical GY6 engine. But from I've read it's pretty much the same. Measure and double check if you order parts. The studs and nuts holding the exhaust pipe to the cylinder head. I'm guessing maybe someone had the exhaust off for what ever reason, I think mine had to come off when I removed the back wheel. Anyway it's just a stud and nut, lots of heat/cool cycles and vibration. It's a rather common problem for these to loosen over time and it's not one of the easier locations to reach when the scooter is setting on it's wheels. Not hard to get to but awkward. Tighten the nut "just right" and you shouldn't have problems. "Just right" tight enough to hold forever but lose enough to not strip the threads or break the stud. Actually you ought to torque it to specs and of course don't use an impact wrench for tightening. For what it's worth on a 50cc scooter I have I made a copper O ring from some soft solid copper wire for an exhaust gasket. The way the pipe was welded to the flange on the after market exhaust happened to make a perfect seat for the O ring. So far it's worked just fine.