Replacing the valve adjusting screws with the 911 type requires that you replace the pushrods as well. This is because the 911 screws are a little bit too long. This is not a drop in replacement as there may be some additional modifications that need to be done. The best solution is to replace your screws with stock original Porsche ones. Aftermarket screws have a tendency to be too hard, wearing down your valve instead of your valve screw.
Screws are much easier to replace than valves.
I get closer but nowhere near exact. The reason I ask is I am attempting to install A Crane XR3000 ing. Sometimes the paint gets worn, or more likely, the ring gets rusted and the paint falls off. It's pretty obvious where it should be. One tip, never turn the engine with the fan blades, despite what anyone else may tell you.