Thanks sperrysymc! I did that and copied the kernel extension back to the system folder.
Nevertheless the system doesn't boot. Maybe the access rights are wrong now. And the Single User Mode tells me that it can't load a dozen of files, not only that one kernel extension.
As I am not familiar with the terminal and how to change access rights there, I will now try to install Snow Leopard over that FireWire connection over the old system. I hope that will work and repair enough to make the machine boot again.