How do you scale up a kit?

Do you use a percentage? 10% more wing cord and 10% more span? And 10% longer fuselage? 10% larger tail surfaces?

I had some basic design ratios once, but have lost the file. Somebody had pics posted of span/cord ratios and span/length ratios. Ratios for square inches of tail surface too wing area.

I have a kit that I want to do this with. I want to keep the fuse width the same so my cowl still fits, but I want to stretch the overall length of the fuse and wings to accommodate a larger engine (gas). Not sure if I should make the fuselage taller too?