The question of putting 1.6 heads on 1.9 blocks comes up once in a while. In short, it shouldn't be done. The problem lies in the small displacement of the 1.6 head's swirl chambers, specifically sized for the displacement of the engine. It has been a while since I've done the math, but it turns into something like a 27:1 compression ratio when used with a 1.9 block! Although, it would be interesting to see how such an engine ran.