Also, spares can be horrendously expensive because of scarcity. As an example, some years ago we had to replace a fuel tank. This is just a rubber bladder, but it cost us £4,000 for one! In more enlightened countries, metal fuel tanks can be used to replace the inferior rubber ones at much lower cost, but as these are not 'original equipment' on the type, the CAA will not allow their fitting!
Is this not so with many aircraft and certain spares which are scarce. And I seem to remember some serious costs getting a PA28 fuel tank resealed. Not at the £4k level sure and not a scarce item, but expensive enough.
The point you make shows the continuing idiocy of compelling the fitting of "original" equipment when there is something cheaper, more modern and safer available. There is no virtue in the term "original" and what was original was often a compromise, or something fitted because it happened to be available. This is not something permit types suffer from and as we know they are not falling out fo the sky.
The chippy isn`t a looker
Are we talking about the same aircraft? "Looker"? I think sitting on the grass outside the hangar on a warm summer's evening there is little that looks better in the aviation world. And unlike some that just look good the Chipmunk also flies well, as many far more experienced than me have repeatedly testified to.