to restore yaboot you need to boot into linux and run:
ybin -v
you can boot linux by inserting a ubuntu install/live cd. you then might have to mount your old root partition and then run "ybin -v":
mount /dev/
mount -t proc none /mnt/proc
(If /usr, /var and/or /tmp are on separate partitions, mount those too)
chroot /mnt
ybin -v
you can also boot into linux without using a linux cd rom. you can boot into open firmware by holding down the "option" key (which is the "alt" key to the left of the space bar. once in open firmware, click the linux partition and click the right arrow button "->". once in linux, type "ybin -v".
more info about open firmware commands and special keyboard key names can be found at http://homepages.gold.ac.uk/suzanne/startup.html