Some FSX aircraft can port over to msfs 2020 using Legacy Importer at Nexusmods. The Calclassic DC-6 and 7, FSX DC-3, and Convair 340/440 will work but not the Constellations, Lodestar, or Convair 580. You don't get a working panel and you have to edit the aircraft file to add more h.p. for take off.
I just got started with L. I.so there may be many more that will work.