yeah i am an airframe mechanic who was never able to work on airplanes... stupid 9-11 anyway they are my passion and where i don't know much about sport planes i know a fair amount about the concept. one of the things you can do although it may look weird is to add winglets. I do know that they "extend" the wingspan by a foot or so by limiting the wind slip over the end. especially with a wing profile like that you can capture the full effectiveness of the wing without compromising the structure at all. if you are going to address the structure to extend the wings you need to make it much stronger than you think or it will crumple under heavy stress (acrobatics) if he is just looking to fly the thing without whipping it around it won't need to be reinforced. also do you know what kind of construction the wing is currently made of if you could get a picture of the inside it would greatly help me to make suggestions to you as everything i say now is just nice in theory.

it's best to live a full life first, but to go out in a blaze of glory that would be bliss
Alden