If your challenge is getting the "dog bone" out, yes, it can be difficult. The magnesium case just loves to galvanically corrode to different metals. There is a circlip retaining (7) the steel (old) or aluminum (new) button plug (9).
Remove the circlip, or more likely excavate what remains of it. Use some heat from the outside to warm the case around the plug, and tap the plug out with a long drift, working around its circumference to the extent possible. Install a new, uncorroded plug and o-ring. Last I checked, the alu versions of the plugs were available from Porsche: I have a few of them in stock.
I hope this helps. Patience and some overnight soaking with penetrant will help some.