before anyone tells you to use the search function, or tells you to save your money, im going to!
it doesnt reduce vibration, it INCREASES it. rather than constraining the engine, its allowed to flop around wherever it wants to go...bad idea!
mount the engine as solidly to the frame as possible. if they had 3 mounts, ok, id advise differently, but with two...you need them TIGHT and SOLID.
welding, or brazing the mounts on so one bolts the engine in tight, and no longer resorts to bolt tension to hold it in place by "squeezing" the frame, reduces vibration to virtually NIL.
by the time you tighten the clamps up enough to hold the engine steady against the chain tension when in use, the rubber has all but been squished solid anyways. defeats the whole purpose. thats why 3 mounts is ok for rubber. triangles are very stable.the bolts can be loose in the rubber, and then vibration is absorbed.
a bird sitting on a wire has to dig its claws in to stay "up". a bat just dangles. the engine on your bike has to dig ITS claws in to stay "up", also.
(hmmm, im gunna have a go one day of sticking me engine in upside down!)
larger frames absorb more buzz too. a stiff bmx frame is just that, stiff. a 24" frame MTB has lots of "absorption" qualities. a 24" MTB frame is also pretty BIG! (18" is sort of standard) this is the seat tube measurement, not the wheel diameter, be aware
whatevers left can be eliminated by using those big fat squishy handgrips, etc etc.
my roadbike sends my fingers numb after ten minutes, whereas i can ride the pushy for hours, no hassles. its the potholes and catseyes that hurt!
bob weights in the handlebars will also reduce any high frequency "buzz".