There's no way a Merlin would fit the Whirlwind without re-designing it into a completely new design. The heavier Merlins would shift the center of gravity too far forward, necessitating a fairly hefty stretch of the rear fuselage, new, bigger, radiators would mean a total redesign of the wing inner sections. And then there was that other Twin-Merlin design. Mosquito or what ever it was called!
Stein