Water temp and oil pressure gauges.

Drilling and tapping is relatively easy. If you drill with a vacuum hose on it, you will minimize the amount of metal going in to the water jacket, but any metal shavings there won't hurt anything and eventually rust away. Mechanical gauges will have to have a larger hole, but.. If you shop carefully, you can likely find an electrical gauge that uses a smaller sender that will just screw into the factory hole.