I do not have a 2 stroke bike or kit, but with how you mention that after passing 25 an reaching 30 an above vibration is less or gone, there is this. Once with a car I had a heat shield on a muffler that a 50 it would rattle and above it was gone. I found the crack in the shield and replaced it Problem gone for a while. It is called sympathetic vibration due to resonance. The reason it happened after I replace the cracked part was it cracked again. Maybe inferior part manufactured or that the bolt hole were not drilled right and caused stress and cracked the part and would continue to. Ah and I noticed this is continuation of an old thread, though others might know something Anyway thin sheet metal that is not secure is possible, chain I don't use have a belt. A short piece of chain I have on a DIY build that is from clutch to jackshaft only.