It should be matter of running
grub2-mkconfig --output=/boot/grub2/grub.cfg
from some kind of rescue environment, eg. after booting Fedora installer ISO with the rescue parameter. IIRC there are versions of petitboot that do not support BLS directly and versions that do support it. So knowing the details about the initrd load failure would be useful. The usual "make backups, etc." applies before doing any changes :-)