Here’s what I have so far that seems to work, you can’t delete your reply by hitting unlike, but you can with the delete button. This starts at line 310.
/* Add like as reply instead of new post in activity stream */
$comment_id = bp_activity_new_comment( array(
'content' => $action,
'activity_id' => $activity_id,
'parent_id' => $parent_id
));
if ( !empty( $comment_id ) )
bp_core_add_message( __( 'Reply Posted!', 'buddypress' ) );
else
bp_core_add_message( __( 'There was an error posting that reply, please try again.', 'buddypress' ), 'error' );