to get the Mint back to vanilla state , I don't know any other method except re-install.
If you want to install Ubuntu's grub , then boot to Ubuntu and open a terminal and then give these commands
If you like the gnome2 environment then consider to install Ubuntu 10.04 LTS (is still supported until April 2013)
sudo grub-install /dev/sda
or you can use a new version like Ubuntu 12.04 LTS (support until April 2017) and use the gnome-classic Environment.
About Ubuntu releases - support
Download Ubuntu 10.04.4 LTS
Download Ubuntu 12.04 LTS
Gnome-classic in Ubuntu 12.04 LTS