I just gave a quick look at the plugin and in my opinion it would be easier to tweak the way you use it rather than to tweak the code itself. What you want is basically already existing in the ‘Rationg options‘, it’s the heart option that only allow the user to do a like, or ‘up vote’, which is just the same. You then just have to change the picture of the heart for the arrow one and then have a look at the ‘Ratings template’ to achieve what you need.
