I am trying to copy the product details PHP code into a custom block and then display it on the home page. I changed the code to make it point to one specific item number ($item_id=42, for example). but it still won't work. Can anyone help me tweak the PHP code to point to a specific item (with name, photo, description, add-to-cart buttons...) and display it inside a custom block?

The PHP code in the product details is dependent on other scripts that won't be called when displaying a custom block. I'm not sure you can do what you seek. You could, however, write a simple query to your database and bring in the info and images you want to display and do it yourself.