A Spitfire with Merlin engine or a Griffon engine is still called a Spitfire. A Mustang with an Allison engine or Merlin engine is still called a Mustang.
.... And a DHC-2 Beaver would still have been called a Beaver with the Gypsy Queen engine it was originally intended to have. Thank goodness DHC had the foresight to produce the Pratt and Whitney powered Beaver!