RE: PATRIOT BUILD QUESTIONS?
Although the 91FX has been known in the past for assorted issues, one of them was not vibration. It tends to run fairly smooth. Double check to ensure your prop is balanced.
The engine mounts that come with the XL are fairly beefy.
Replacing the mount would not be all that expensive. Hunt around some of the hobby shops - see if you can come up with a set of old Sig aluminum beam mounts if you want to go that route. A couple of companies make aluminum versions of the "world models" type mounts too.
If you want to go to the extreme, you can install a set of dubro, dave brown vibration mounts, or go full blown and install a Hyde mount.