FileMaker Go 13 was removed from the app store on the 23rd September 2016, after v 15 was released, they generally run two versions simultaneously and v 14 is due to be removed late July apparently.
v 14 and 15 should run the app without the need for many changes I have found.
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If you have previously downloaded older versions of FileMaker Go, you can go to the App Store on your iPhone/iPad and then go to Updates -> Purchased and re-download them.... In attached pic you can see that I am able to download any version of Go from 11 up to the current 15.... I still have 11 installed as I occasionally need to login into older FileMaker Server 11 (.fp7) hosted solutions.
However, if you are running iOS 9.3 or newer and your files are .fmp12 then I would recommend v15 in preference to v13 or v14.