Sometimes people try to. Think about this. If there is rot, it began and worked its way from the top. So if you are trying to fix what you see from the bottom how much more is there (more) at the top that you cannot get to to stop or repair? Some people cut open sections of the mid-cowl (69/70) to get to the lower cowl to repair but you really cannot get to much when you are opening a window rather than the whole thing. On a 67/68 there is no removeable cowl top grate, only the top and the bottom. I would not recommend cutting open the top (which is what you see externally) over removing the whole thing. Generally speaking there is no easy fix for problems such as these.