Best way to tweak a function in a theme?

What's the best way to tweak a function in a theme. I've got a really nice theme that uses truncate_post, but truncate_post filters out tags, including italics, which I'd like to keep in the theme.

I could just write my own function and stick it in a plug-in, but is there a more elegant way to do this, in order to get my own truncate routine that doesn't strip tags?

Thanks!