Sean, a great way to add to your consist is with MTH cars - MTH makes 15” ATSF passenger cars in their Railking line that are very nice. They are literally identical to the Lionel cars sold in their RTR sets. You can put them side-by-side and you can’t tell them apart down to even the undercarriage detail. They have to be from the same tools. MTH has a matching full-length dome car which is neat mixed in with the others. These MTH and Lionel cars in combination are a great way to have a large consist without lots of duplicates. You might be able to find them new at dealers like Mr. Muffins. They are also readily available in the secondary market. Before we converted our layout to scale, we had a bunch of these cars in both styles of ATSF and NYC. When we sold them off, we put the Lionel cars next to the MTH cars on the table and they sold fast as buyers couldn’t believe how identical they are.
As to the A-unit, Lionel made dummy A’s in the FT in the same paint schemes. They also made dummy B’s. You might be able to do a shell swap with a powered F3 and one of these, but that would be kinda expensive.
EDIT: After looking more closely at the picture and item number above, it looks like your set is an FT in which case getting a dummy A unit and a dummy B unit is as easy as finding one on the secondary market. They were made to go with the early conventional RTR sets and are plentiful. The tooling and paint hasn’t changed.
