Missing/backfires, drop in gas mileage

I had a new fuel filter put on, all of the parts are original.
It is better but when you accelerate or start up an incline it hesitates and when you press the gas it sounds like it kind of backfires under the hood. The gas mileage had dropped. After you drive it and get it hot when it starts to cool off it makes a loud pop like a gun fire. It has never given us any problem until now.

One more question, I have lost the key to the locking hub caps how do I get another one?
Thanks for any help I am a single fifty five year old woman that knows nothing about cars and waiting to get my disability check started. Every penny is like $10.00 to me.
Help.