Is there any alternative to isolating the vibration of an ultralight engine other than using engine mounts?
You can prevent engine vibrations from being transmitted to the airframe by motion cancellation techniques. Similar in principle to Active Noise Reduction headsets that generate an inverse pressure wave to cancel sound, motion cancellation generates an inverse displacement of a mass to cancel vibrations. 

Sensors determine the vector(s) required, and electronics control the mass actuator(s). These can be added to any engine. Predictable, repetitive vibrations are most successfully treated. Actuators tend to be heavy, so better engine/prop balance and more effective isolators (mounts) are preferred.