I see no particular reason why not to soft mount a gas engine if the exhaust system will allow it.

I use rubber mounts that allow only a tiny amount of movement (+- 0.2mm)
MVVS in their Edge video show a soft rubber mount with massive movement freedom (see picture). So how in the world can warranty be void if you use just the system they show? Beware though: The slender engine brackets are first mounted to a sturdy back plate, which in turn is used to soft-mount the complete assembly. Also observe the loops in the ignition cable, so it is allowed to move.
The only reason not to soft-mount might be, that the Walbro-type pumper carbs can become upset by the vibrations, and fail to perform properly in individual applications.

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