clone and modify a widget

Hello guys,
I have to create a widget that show the last post from only one category.

1 – I copied the code of widget “WP_Widget_Recent_Posts” from default-widgets.php
2 – Paste the code on the file function.php of my theme
3 – change the code you just pasted and inserted
register_widget ( ‘name_new_widget’);

First i have to select only one category, I think it is the function to change
$ r = new wp_query (array ( 'showposts' => $ number, 'nopaging' => 0, 'post_status' => 'publish', 'caller_get_posts' => 1));
++add 'cat' => id_category_to_select
(and it’s right, because i’have tested it whit the original widget_recent_post)

The problem is: The widget is not visible in the widget page into admin.

You can help me?

Thank you, bye!
Andrea

PS. excuse me for me english, i’m italian ??