I applied my own solution a few years back for this very same problem. It worked for me. Everyone agrees the engine is inherently unbalanced and one writer pointed out that the cylinder head swings forward and backward. The causes the somewhat-flexible downtube and seat tube to flex forward and backward. Vibrations travel throughout the frame and cause undue handlebar vibrations and seat vibration. Attachment bolts were loosening and backing out. Lost a few on the road.
I thought, "Stop the cylinder head from swinging back and forth, and the vibrations stop." But how?
Use a heavy steel wire one-eighth inch in diameter and a small turnbuckle. Loop a length of wire starting at the side of the engine, around the cylinder head between the first and second horizontal fin, to around the seat post, high up where the horizontal tube intersects the seat tube but cannot slide downward, back to side of the engine. Open the turnbuckle wide and form eyelets on the ends of the wire threaded through the turnbuckle ends. Twist the tunbuckle tight by fingers only; you don't want to break the cylinder off the crankcase. (The wire now becomes a "noose".) This will pull the cylinder head backward, toward the seat post. Twist the turnbuckle until you cannot rock the cylinder forward or backward by hand. The tight wire harness will greatly limit cylinder rocking forward and backward.
When I did this move to harness the cylinder head, my 4500 RPM upper limit at which I could not tolerate the vibration shifted upward to over 5800 RPM as measured by a tach. At that point, I was traveling too fast for comfort and did not push the engine for more speed.
I tried the isolators mentioned in the postings. Tried deadweights on the frame tubes. Rejected all of them. Only after locking the engine in place, did high-rev vibrations become insignificant. Though I moved into 4-stroke engines, that original 2-stroke engine still has my wire harness in place and intact just like it was over three years ago.
