The Roadrunner was a popular kit back in the 80's. I don't think anyone is kitting one right now. We have a club member that scratched up a set of plans and built one, but it's his own variation. He actually used the airfoil from an Ugly Stik, scaled up to be an eight foot wingspan, and then sketched out everything else based on looks and proportion. The only problem, he didn't even make a set of plans... He flies his on a 41cc Quadra. It's a sweet flying plane. He's an amazing builder, and it looks as good as it flies!!
You can always try the classifieds on RC Groups, or wait for a kit to show up on eBay. You could also start with a set of Ugly Stik plans and modify from there. The tail surfaces will be a different shape, and the fuse is probably a bit longer on the Roadrunner. I think the Roadrunner fuse also tapers top and bottom, not just on top like the Ugly Stik.