The stock mounts look fine to me. The only thing I don't like is that they are not width adjustable, but you should have no problem if you stay close to their recommended engine range. I normally don't care for this clamp-down kind of motor mount, but for a little .46, it should do just fine. If not, you can always drill and tap new holes to bolt the engine down instead.

Yes, they mounting holes are offset on the firewall to compensate for the built-in right thrust. Check to make sure the engine prop hub is centered along the thrustline after you've mounted the engine