There is a paint called Fuzion Fusion...something like that, that is made specifically for painting plastic lawn furniture and stuff. You can get it at Pep Boys and Auto Zone. I have had great luck with it on fenders and other low rub areas. It doesn't withstand the knee braces on the shrouds, but should work well on an airbox. Basic gloss black will flake off pretty quick. Follow the directions and make sure the airbox is as clean as possible before painting.
I use black numberplate background on mine and you might want to use that on the sides while painting the rest.