You might be able to get a hardware store to mix you up some oil-based paint (like Rustoleum) in your color. Then you could just use a foam brush to apply it, (since it's a small area) and color-sand and buff it to perfection. Or I guess an automotive paint shop can mix up something a little better and even put it in a spray-can. If you have an airbrush, then you have options.

It's pretty common for them to rust there. I think it's from moisture condensing between the sunroof frame and the roof sheetmetal - or outside water leaking in, and rusting it there.

My '82 was doing that by the time it was 10 - 12 years old, and it wasn't too much of a rust-bucket.

Back in the mid '90s I saw an '84.5 GT350, and a quarter of one of its A-pillers had rusted away, from the roof seam down several inches! The rest of the car looked fine.