OK now you're REALLY mixing me up. I have NO idea what this thing looks like.

First off anything with a swivelling engine is NOT "pretty much conventional". And the engine is in the middle? You mean the middle of the wing? Also now you want to move the CG BACK by making the stabilizer SMALLER? I think we need a picture of this "conventional" design. Your second post here makes it sound like a canard unless you meant to say larger instead of smaller. In any event making a stabilizer smaller OR larger isn't going to let you live with a bad CG placement that is this far out of place. And making it larger or smaller to alter it's weight enough to move the CG is going to have bad consequences on the flying of the model and is still adding weight just as if you added ballast.

How about a picture or a link to a picture?