Managed to bork my system. Messed up grub. What can I do?

So I had a lovely Fedora 32 system running nicely on old hardware (Dell Dimension E520). For reasons I won't bore you with I wanted to try out Manjaro Linux, but didn't want to install it. I created a bootable USB and started it from that. The problem was that Manjaro doesn't seem to allow the creation of persistent storage on a USB.
So... My brilliant solution? I have a spare 32gb USB stick, I'll just install it on that. What could possibly go wrong?...

Well it installed OK. It even boots from the USB.

The problem is that now I can't boot my Fedora 32 system.

The Manjaro boot screen shows a Fedora 32 option, but selecting that just leads to hundreds of errors.

I can put a Fedora 32 (Server edition) DVD in the drive and go into the rescue option on that. It shows some errors, but does eventually get me to a command line.

What do I need to do next? My files seem to there under /mnt/sysroot - but I am unsure as to what best to do.
I have seen these instructions https://docs.fedoraproject.org/en-US...ng_GRUB_2.html

and also this page from the same site: https://docs.fedoraproject.org/en-US...ng_GRUB_2.html

Which do I need to follow? And do I need to chroot or something before I do so?

All help gratefully accepted...

Thanks!