A number of modifications were made for the Goose, but the most numerous are those by McKinnon Enterprises of Sandy, Oregon, which holds 21 supplemental type certificates (STCs) for modifying G-21-series aircraft and which also manufactured four different conversions that were recertified under a separate FAA type certificate as brand-new "McKinnon" airplanes. The first was the McKinnon model G-21C which involved replacing the original R-985 radial engines with four Lycoming GSO-480-B2D6 piston engines. It was approved under TC 4A24 on November 7, 1958, and two examples were converted in 1958–1959. In November 2007, Antilles Seaplanes of Gibsonville, North Carolina, announced it was restarting production of the turbine-powered McKinnon G-21G Turbo Goose variant, now identified as the Antilles G-21G Super Goose. Pratt & Whitney Canada PT6A-34 turboprops flat-rated to 680 shp (510 kW) would have replaced the original PT6A-27 engines, and the airframe systems and especially the avionics (aviation electronics – i.e. radios and navigation systems) would have been updated with state-of-the-art "glass panel" instrumentation and cockpit displays. However, as of 2009, Antilles Seaplanes' manufacturing center has been foreclosed and sold at auction. The fate of new Goose production is currently unknown.
