SAP B1 is small to mid-size business ERP application, and please try to tune your expectations to simplicity and reasonable customizability of SB1.
If it doesn’t fit exactly to your business processes as they are in your legacy MRP, then you should adapt as much of SAP BO standard logic as possible and only reasonably small circle of required custom features should be implemented via modification and programming
1. Serial Numbers for Warranty contract item. You can have either your internal or manufacturer serial number.