Ok well here is where my question  gets a bit hazy. If I go with an .09 or .10 that should be close to the right size. If I go with a .15 it might be a bit large but I'm guessing that by prop and venturi selection it can be run at what ever speed needed to fly The  model at the right speed. The weight difference is less than an ounce I belive.

Aside from adding weight. Couldn't wing position be moved to adjust the balance within reason.  I'm not talking about drastic changes here. And I know weight considerations are an issue. I'm not aiming to build a rock on a string. Though I could end up with one. I'm just trying to find ouf if a slightly larger engine would be able to slowed down/or controlled to fly at a slower speed through the use of prop and venturi size.