Changing boot parameters on Debian standard ISO

Hi,

My 15 year old son has found an old 2011 iMac in the side of the road. Long
story short is that the processor probably works, but we think the GPU is
dead as we cannot get video out of the mini display port connector, or the
screen.

So, I would like to boot from a Debian live ISO with the following kernel
parameters (set in grub.cfg).

    console=ttyUSB0,19200n8

Now, I can mount up the ISO in loop mode, and get into grub.cfg, but how do
I write this back to the ISO so that I can put in on a USB flash drive?

Thanks!

Paul.