Whoa nelly! It sounds like you really don't need to scale up your entire aircraft just make it longer. if you scale it up your cowl wont fit.
Basically I would need more information as to what you have, then, what you want. what kind of plane? is it a kit? what kit?
the more information you give the better answers you will get. Basically it sounds as if all you need do is add wing tips and move your empennage rearward. or again maybe not. we really need to know what ya got and where you wanna go with it