It sounds like you have set the store to not track inventory. If the store is set to track inventory then out of stock products will show as "out of stock" and ordering is not allowed. When you set the store not to track inventory products are "available for sale" and not "out of stock".
So if you want to use your current setup it will take more than just a simple change in this template file.
Or you can use this module -
https://market.x-cart.com/addons/backorder-preorder.html