"Call to undefined function fivestar_widget_form()" error
There's no function called fivestar_widget_form() in the Drupal 7 version of Fivestar (I've just grepped the module folder).

It's likely that function is a CCK hook remnant, CCK having been incorporated into the core Field module in Drupal 7, and all of it's functions renamed.

As far as I know there's no easy way in Drupal 7 to use a field widget form as a stand-alone form so you might have to look for another way around this problem.