According to the ICAO aircraft type designator lookup tool, all Kitfox models from the Skystar or Kitfox companies use the code "FOX" with the exception of the Series 5 Vixen and Voyager (naming evolutions of the same tricycle gear airplane), which use the code "VIX".

There are no entries for the early Denny Aerocraft models, so I would just use "FOX".