Hi Maxime:
I suppose I was waiting for those more qualified than I to speak first. Filz Tschapka scare me to death, and this example is no exception. From what I can see (important qualification), the Tschapka appears okay from the outside. However, it is difficult to judge the profile of the helmet without a dedicated side view.
Two large concerns, IMO, are the liner and the Preußen Infantry Wappen. The liner, with its rounded tongues, looks like it was "borrowed" from a conventional Pickelhaube, and its bunched appearance would indicate it is too large for the Korpus. Due to its proportions, and the absence of interlaced breast feathers beneath the "FR," I believe the eagle is also from a Pickelhaube.
Externally, however, the Tschapka closely resembles the specimen on page 385 of the Kraus reference. This could be an original helmet with a replaced liner and eagle.
I don't think a definitive endorsement of this Tschapka is possible unless someone out there owns one and cares to participate.
Chas.