Here is a thread about converting a small screen to run on 5V. You would really have to know what you are doing to accomplish this:
http://www.raspberrypi.org/forum/viewto ... 1&start=75
Generally is is much easier and more efficient to step down DC voltage than to step up. If it were me I would use a 12V battery for the screen and then step down to 5V for the RPi using a 'buck' or switching type regulator.
You can get a simple cheap converter by using a car USB adapter with a 12V battery.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.