What is the value of a 1964 penny with a small bust of Kennedy superimposed on its face?
This is a standard US cent stamped with the Kennedy bust after leaving the Mint, probably as part of a rash of products to honor JFK and/or to show the alleged similarities between the lives of the two Presidents. The etching/stamping actually ruins the collectible value for most coin collectors, although there may be some after-market value for those few fans of these coins who don't already have one.
Most likely the image you see on your penny is John F. Kennedy. These Lincoln/Kennedy novelty items were popular advertising items back in the 70's. This is not something done at the Mint. Somebody took a stamp and manually counterstamped Kennedy's image onto the penny after it left the mint. These novelty items generally sell for about a dollar. Dan
What is the value of a so-called Kennedy bill with Jackie and JFK superimposed on a 1 dollar silver certificate?
If your Penny is a small coin of about 11 mm diameter with a crowned 1 on the reverse, it is a Maundy Penny and might fetch anything up to £20 GBP if it is in mint condition. If it is a regular penny of 30.81 mm diameter with the seated figure of Britannia on the reverse, not much. Modified coins have little if any collector value.