I tried "rubber" mounts for my motorized bicycle early on during it's prototype phase, but as the engine still wasn't broken in and was 4stroking a lot I found that although the mounts helped reduce the vibration I felt - it accented the engine's shaking and contributed to a fuel frothing problem and making it run even worse.
No - I wasn't using old bike tires for mounts or anything, I had "discovered" some nice polyurethane transmission racing mounts in the "other" pile at work and made bushings out of the cut down stock.
Only 1/8" thick I didn't think it would cause any problems, but man the motor ran a LOT better w/o them... I suppose now that I've gotten it running sweet I could put them back, but the vibration isn't really bothering me anyway what with the comfy gel seat I now have etc.
Just a thought