Complex sentence: Free of some of the problems in the certified area, and populated largely by experimenters looking for better performance, the kit market exploded in the 1980s and had many designs that far surpassed the performance of certified offerings, while also being much less expensive.
Split into simpler sentences: Free of some of the problems that certified aircraft had, and populated largely by experimenters looking for better performance, the kit market expanded rapidly in the 1980s. Designs available as kits often surpassed the performance of certified aircraft, while also being much less expensive.