This subject comes up often and there isn't a good answer. There are a lot of opinions however. Some will say you must use high temperature paint and others have used cheap rattle can stuff, all with good results.
Personally I've used both and have had good luck. I like to color match the engine with the bike and have used Dupli-Color automotive spray paint and never had a probelm.
Some paints are not compatible with gasoline so you need to keep that in mind. If you want a good fuel proof clear coat over the color I've been using Top-Flite model airplane clear over paints that won't take gasoline. It's available at hobby shops.
Whatever paint you choose you need to make sure the engine is clean and oil free or no paint will stick. Use a good solvent to clean the engine before applying any paint.
Just be patient. You'll get other opinions and suggestions on painting the engine.
Good luck and show us your results.
I use dupli-color to paint the Honda engines I restore and it is a good match for the silver. As fat as black, you need to read the can because some of them require you to bake the paint on after being applied.
I use use a heat gun to bake on all my paints. If you paint a frame with high heat paint you should bake it on with heat. This Is how it cures and hardens. Everyone has their own way this is mine.