In AX 4 and 2009 if a purchase order (PO) was deleted you could easily navigate to the voided purchase orders and restore the same.
But since AX 2012 was released there is no longer an option to restore or view such Purchase orders.
I strongly suggest you however setup database logging for the Purchase orders header on delete that way if a user does delete it you can view the same from the AX table SysDatabaseLog as show below:
This will just help track who has deleted the same which will help in an audit sense.
Keep on DAXing ppl.