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.
As you've noted, engine mounts are used to isolate the vibration of the engine from the airframe.
Another technique is Dynamic Prop Balancing - adding weights or washers to specific places on the propeller or spinner to eliminate the vibrations.
It's the same idea as is used to balance car tires.
Perhaps the choice of the engine helps, flat-six does vibrate less than in-line engines, also Wankel Rotary Combustion Engines have an intrinsic very low level of vibrations.
Some interesting approaches were proposed, as this patent from the extinct Barcelona Based motorcycle maker: 'Sanglas'. They once prepared a working prototype for a single-cylinder, 4-valve, 750 cc engine.