You need to be very carefull with piston to valve clearance.
And the timing chain tensioner can only take up so much slack too.
Another option...
If you have a Non-SVT you could install the SVT pistons.
Another option...
Have material welded (added) into the custion chamber. Of course this means fully custom porting work too...
If you have a Zetec. Check into "Focus" aftermarket pistons and other guides for limits on milling the head
All of them would be labor intensive and some cost intensive...
