Hmm, for little scratches like that... If you're already airbrushing, that would be ideal I suppose, but I know the deal... set up, mix the paint, clean the airbrush, clean up etc. ... it can be a pain! (I live in a townhouse with no "modeling room" like a lot of the gang here, so anytime I'm working on something I need to hijack the kitchen table!!) If you do go that route, you can use a sheet of bristol board with a small hole cut in it to make sure you're not overspraying.
But, If you want to stick with brushes, make sure you have allowed the airbrushed coat to dry at least a day. I know acrylics are dry to the touch after a few hours, but if you put thinner (alcohol) on paint that is too fresh, it will start peeling & you'll have other problems. So with that being said, go ahead & use the paint brush, but you don't need the paint super-thin, or it may run & not stay in the scratched area. I'd use a small brush & paint in the scratches as carefully as possible. Leave it to dry again, then use the absolute finest grit of sanding film you can find. The area you brushpainted will likely have a little bump - sanding will help even it out with the airbrushed areas. Just take your time, and it'll work.