Huh, we only have two years warranty here. Mine ends in a week
As the others tell you, seems really stupid to pay for extra warranty time since you made modifications to your car. Bmw would ofcourse blame the mods when something happends.
To me you already rolled the dice when you modified it.
I dont think your local dealer can pay you a brand new engine by themself.
If you are so sure they support you get it in written form.