Most spray paint will work IF you thoroughly clean every last bit of the engine with prepsol or a suitable solvent.
Use a self etching primer, then spray with several light coats. These being aluminum engines with cooling fins, can successfully be painted without high temp paint.
Just don't try it with the exhaust.
Everyone who I have seen paint these successfully have use high temp engine or tractor engine paint. Let's face it the engine gets hot. If you are going to paint it you should really use paint made for that purpose. You can use a 2 X 4 as a hammer but if you can get a hammer wouldn't you rather use it.
Power Coating would be nice but most of us sure don't have the stuff to do that. But the same store that sells regular spray paint also has the heat paint you might as well go with best suited for the job at hand.
Keep one thing in mind. When you paint the engine you in fact are actually going to make it run hotter.
The aluminum is full of micro pores that you will be sealing up and that does make a difference.