Let me add another example to clarify what I meant.
P-47D-40 already made, P-47N already made. Put the N engine on the D40 frame, done!
But on 109E-7, for example, you need to model a new engine entirely, which means new power curves for speed and climb, needs to have the spinner changed, and a couple other minor issues. It's not something already-in-game, that they can cut-and-paste together.