At a guess, I'd say its going to be more about the prop than the engine.
Since more power basically just stretches the power available graph upwards and leaves the power required graph untouched then the point with the most excess power isn't going to change very much.
But if you change the prop, then you may move the 'hump' in the power available graph left or right and therefore change Vy.
However, if you change the engine you'd probably need a different prop?
pb