Jorge,
I watched the video and have seen that in my engine. I fixed it by replacing the 1. worn carburetor sockets (rubber part at the flange)
2. adding the rotax recommended tension spring from Carb to manifold. (the engine in the video is missing the spring. It dampens movement)
3 repaired the cracked engine mount.
your mount may have a crack you haven't found or I suspect your prop is out of track or if ground adjustable, has a blade out of angle with the rest.
when you changed engines, did you keep the same prop, prop flange, gearbox etc?