You might want to check out the Morgan Aeroworks designs from Australia. It's a well-established company not well known outside Australia producing straightforward fabric-covered, pop-riveted aluminum tubing designs with premolded fiberglass wing ribs, cowlings, fairings, etc. Their Super Diamond four-stroke single-seater can take many engines in the 60-80 hp range and they have a new Super Diamond Twister model coming out that is approved for aerobatics. The U.S. dollar is pretty strong against the Australian dollar these days so the complete kits less motor, instruments, paint and upholstery run about $14-18k for the single-seater though I don't know what shipping or import duties would run. Here is one of their two-seaters uncovered so you can see the construction method.