Putting a piece of rubber behind the engine pount won't reduce noise, because there is still "hard to hard" contact (the mounting screws).

If you want to achieve noise reduction this way you have to completely isolate the engine from the airframe.

Try this: http://home.online.no/~ofremmi/HowTo.../HydeMount.htm
This "Hyde mount" is mostly used on pattern planes, but can be scaled down for your application... it will just take a little experimenting.

A properly set up (calculated) mousse can pipe is a sure to way to succes, and will even increase engine power.

Like other people said, induction noise from the carb may not be overlooked... it is a much bigger noise factor than most people realise.
There's absolutely no use in quieting everything down if you do nothing about the carb... since the human ear will always pick out the loudest noise.
Try to get your air from inside the fuse one way or another, that will do the trick. Just don't use a hose the size of you throttle opening all the way to the fuse, that's too restrictive and will kill performance.

I seriously doubt you can make is as quiet as a brushless though... unless there's something terribly wrong with the brushless you're comparing it to.

Finally, I do not own a diesel engine, but I'm looking into it. If everything I've read is correct, than a diesel is probably the easiest way to really quiet things down... but you will still need a flex mount of some sort.