Hi Acam,
What Ulimate are you going to use? The CG Ultimate has 54" span and 980 square inches area. One of the guys put a homelite 25 in one. It is a four pound motor. A trimmer engine has a poor power to weight ratio, so it can't be considered high performance. Most of the best performers use electronic ignition to save weight. I plan to put a Mac 21cc on EI in my CG Ultimate. It should come out at 1.5 pounds per square foot for a ten pound airplane. You can do a search for the Homey powered Ultimate in the conversion forum. Here is a picture of it.

Dave