As I recall, core osC for that version does not provide an "available stock" display field on the product_info.php page. I'm sure there's a simple add-on for this. When you locate it, just wrap the available stock code using the same IF statement.
Ok, ..so I don't have that version here. The closest to this that I have is v2.3.4
I ran a quick scan on that version and the results were slightly different.
advanced_search_results.php
index.php
/includes/modules/product_listing.php
That product_listng module is included in several files.