[REQUEST] Reduce a high polygons model

Hello,
I'm far from being a modeler, I'm just a simple gamer who is looking to improve adding content to the game (modding GTA V to be precise).
I wanted to import this helmet model that has been kindly shared to the community: Shoei Neotec2 motorcycle helmet
But I have a problem: the model is a kind of "scan" and therefore has several MILLIONS of faces (and is therefore impossible to import in game). My goal is to get down to the minimum number of faces without losing quality (about 5000-6000 faces). One person has already succeeded in doing this but keeps it private.I have already tried but each time the model "breaks": the curves break into thousands of peaks.
I wanted to know if someone has already done this kind of thing?
Thanks in advance !
The original model has around 1.5 million faces after basic fixing. I don't you can even get a basic contour to be intact if you reduce them to 6000 faces. 60000 faces maybe. but 6000 is way too less. I would like to see the model of person who did in "private"
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Alex, typical modeling software dutifully reproduces the unwanted bumps. One would save time by using this model as a template to build a low poly model from scratch. Less headaches.
I understand, thanks for the answer, too bad I have to start the 3D again but it seems I have no choice