The pistons are different for the 2v vs.the 4v heads, the have different valve reliefs cut into them, also the 3v heads have their own special pistons. it would not be worth it to swap the heads and pistons for just 40 hp. I would stay with the 2v heads and nurse it for as long as you can. Pick up the complete navi engine and fix it, maybe even hop it up some and have it on standby for when your current engine goes caput. If you have to drop the pan and swap out pistons to do a correct 4v head swap, it's the same amount of work to just fix the navi engine, unless the block is damaged.