Doom classic source code for iOS was released by id software, and has a download link on this page: http://www.idsoftware.com/doom-classic/. The issue is that the url doesn't work, I sent an email to their support but no answer. Do you know an alternative source to download the Doom Game for iOS?
You can get it from "http://www.doomworld.com"
But getting it from idsoftware directly would be better. I hope they will read your mail and fix the problem.