What would be a good hard clear coat to use on an engine? And can you spray it over regular high temp engine paint.

I can't believe someone has not chimed in yet.
As far as a clear over engine high temp paint you need to find a clear that is compatible with that paint. You can go to a good automotive paint store and they can help you.

I like emron if you prepare your engine and paint it with 5000 or 6000 emron you can't go wrong. The 5000 does not need clear coated the 6000 needs a clear coat. I painted my block with 6000 black, and clear coated over that. I painted my valve covers and pulleys 6000 red. I painted my aluminum intake with just 6000 clear and some gold sparkle. To really make things nice after the 6000 red I mixed gold metallic sparkles with 6000 clear and shot the mixture over the red. Then came back and shot straight clear over that. Came out real nice you can mix any color sparkle or any amount of sparkle your preference. Just remember it is easier to add more than take it off so start with just a pinch of sparkle you can always add more.