Rebuild on old shower controls that (probably) don't have today's minimum requirements is, IMHO, kind of a waste of time and money. Yes, you can legally, per code, rebuild an existing installation in most places, but it's better in the long term to replace the whole shebang. It is rare that someone makes trim that fits older faucets. Not saying it doesn't happen, especially if it was a particularly popular one, but there isn't a huge market, and the variations makes it uneconomical. The remodel plates generally required to cover those holes when a new one is installed don't look bad (would look better than the gaps you have!) and would work until you decided on a remodel and can do it without one.