I've allways wondered why there isn't any rubber or anything on the engine mount of my kart. It's just metal on metal. Would putting some rubber in between there help with vibration?
Don't use rubber. You may suffer a slight power loss. You may toss your chain from the alignment varying. You may shear your engine mount bolts from the additional forces, since they can't possibly be tight enough. -or-
It would to basically nothing, enjoy the sensation of riding a kart without suspension or design yourself a buggy, don't go out of your way to make your kart without suspension into one that is.