I to have a lot of vibration at high speeds. It's a new 80 cc motor I bought on Ebay. With shipping it was about $110.00 . The bike is a old cruiser I picked up at thrift store for $37. Don't know the model, it might be a Target or K mart special. I strip it down and repainted it and added new tire. I also had the rear tire trued up but there is still a little misalignment. I don't think the vibration comes for the bike but from the motor itself and the misalignment of the sprockets and chain.
One thing I did when I installed the sprocket. I measured the distance from the axle to the end of the teeth at four apposing positions, making them equal. I also measured the distance from the bike sprocket to the large motor sprocket at different positions and tighten it so that they were somewhat parallel. I don't see any other way to align the sprocket. I'm also thinking of a way to make the small sprocket on the motor inline and parallel to the large sprocket. I'm not positive that all these things will reduce the vibration but if you think about it, wouldn't you want all these things in a true position to each other and balanced. It may be difficult to do sense the tolerances on these part must be wide open but you got to try.
I'm also going to install motor mounts but I'm still researching the engineering. I read that it's better to install one motor mount and making the other side ridge so the engine will stay inline. Or would it be better to instill two mounts. One on each side. I guess it may depend on how hard the rubber is.
I welcome any input.