I finally get some results, at least I got a booting (I mean: it doesn't hang during the bootstrap), patched kickstart. But nothing is easy, so it doesn't boot neither from HDD, nor from FDD.
I'm trying to understand if the problem is becuase my hard drive (FAT32) isn't bootable... It works fine with the stock Kickstart and the "minimal boot" floppy (the workbench is on the HDD but it uses a FDD to begin the boot). But it doesn't boot using the modeed kickstart.
Maybe it doesn't like the FAT32 based HDD ? Or some files are missing?