what file does my airbrush art have to be for silkscreen

You should ask these questions to the printer you have chosen to do your job. All have different requirements based on their individual setup.

Certainly they would want as clean an image as they can get. So airbrushing on metal would probably not be recommended. The idea is to get as transparent background as you can, so unless the metal will be a part of your design texture, don't use it.

As splathead said the printer you are working with would best be able to answer your question.

With that having been said, JPG is really not the format you want to use. JPGs offer only a scant 72dpi of resolution and for serious print work 300dpi or better (at your final print size) is standard.

When you scan your art you should be able to select the file format when you go to save the file. I'm guessing the problem your printer is having with your file is that it is a JPG which does not have sufficient resolution.

If you scan your art so that it is 300dpi (or better) at the final print size and save it as a TIFF you should be golden with most any commercial printer.

Okay, reading this over again it sounds like you've taken digital photos of the metal piece and merely pondering if it would be easier to redo it on paper to scan. In my response above I was thinking you were scanning from the start. My bad. Scanning will give you better results but whether it's worth redoing what you already have is a tough judgment call.

How big is the JPG you sent to the printer? Did you reduce it at all or is it directly from the camera?