I don't think you need to go to 30% or even 25% enlargement. 20% will give you an 8 Ft, wing and 1555 sq. in. area, and if you can keep the weight down to 12 or 14 lbs or so, it will float like a butterfly. Just doing the cube function on the 20% increase on 7.5 lbs gives 13 pounds. and the Ryobi weighs about 4 pounds in itself so a 12 to 14 pound target seems reasonable. When you scale up the plans go for pine or spruce spars with shear webs, good strong struts, braced tail surfaces and stout landing gear.

My 1/4 scale Cub is very close to the size of your 20% enlargement, and flies fine with a 28cc Ryobi that turns a 18X6 at 7000 rpm max on a good day. And the Cub weighs 17 pounds!

Allan