And can i download somewhere the OS with the Dell custumizations ?

No

Why not? There should be a way to create the recovery usb stick through some app like Dell OS Recovery Tool... however it doesnt seem to work (at least for me).

I have a Dell Gaming 7567 that came with UBuntu 16.04, which I removed to install Windows (I own a copy) and then reinstall Ubuntu with dual boot. However I cannot do this since I lost my key with the recovery Ubuntu image. So I tried the Canonical Ubuntu and surprise! It doesn`t work correctly. I have problems with the login screen and it keeps crashing. I also cannot turn it off correctly since it also crashes and I have to press and hold the power key.

So, yeah, there IS a way to get the iso, but someone is not doing her/his work correctly and thus I cannot download the image using Dell`s official app Dell OS Recovery Tool which only purpose is to make bootable USB sticks/CD/DVD with  an OS image.

Cannonical Non-dell customised UBUNTU 18.04 Bionic LTS (stable)

Link with latest updates pre-installed:

http://cdimage.ubuntu.com/bionic/daily-live/

U will see a folder with today's date, open it (DONT OPEN YESTERDAY"S FOLDER)

Download file : bionic-desktop-amd64.iso

These are generic ISOs released by Canonical, and works pretty well on  my system.

Dell customised Ubuntu images are not listed in their servers, hence u will be unable to dwnld ( even if u had ubuntu out of the box)

I am in contact with Advanced Resolution Team & they're on it!!

For now, satisfy ur thirst with the link I gave^