That's exactly the same as the sentence you originally posted, except it has 私は at the beginning. に marks destination, so you place it after the place you're headed (in this case 京都). You don't place it after temporal nouns, such as あした.
あした東京行く might be acceptable colloquially or in some dialects, but it's not really Standard Japanese (although Toritoribe's comments make me think it's just not acceptable.... well, to him, anyway).
Here's the lowdown, in case it wasn't clear from the way it's been worded to this point: あしたに -- no; 京都に -- necessary (and it doesn't matter if there's a time word in there: you still have to mark it with に to show it's the destination).