Just use the advanced editor. Instead of adding scripts to each page, use the Global Page script.
Just add to the script, so it checks if your item is held. If it is, it tacks the text "Ammo Counter: 100 / 100" to the end, where %AMMO is how much current ammo you have, and %MAXAMMO is the total ammo you could have. Or something like that. This way, it only shows it on the page, at the bottom, if you have the item.
IF ITEMSTATE1 = 1 THEN
BEGIN
$PAGETEXT := $PAGETEXT + "<br>Ammo Counter:" + %AMMO + " / " + %MAXAMMO
END
You would need to make it so that on the item, when it is used, it subtracts 1 from the ammo variable.
As for score - why, you just add to the text of your last page,
Your final score is %%SCORE%%.