Is a single cylinder go kart with no suspension. Vibes are part of the experience but I'm understanding where your comming from. I had a 1999 Manco Dingo 6 SE and it had a 6hp tech OHH60 powersport engine and it had one **** of a vibration just above idle.
Switched to a genuine Honda GX 200 and no harsh off idle vibes to speak of. Some small engines are worse than others.
Rubber mounting a engine can be tough because under load the engine will deflect and could cause driveline alignment issues and or further vibrations from misaligned drive components.