You want to know the best way to make it look nice and crisp?
Recreate it yourself. Use the artwork you find as the basis and (assuming nose art for example) create about a dozen or 2 or 3 dozen layers using lines, arrow tools, make those into selections and fill with bucket, along with polygon lasso, etc, and make it from scratch.
Just, save with multiple iterations of your filename, and when you're done save a copy WITH all your layers. Then flatten all the layers. Save again as a copy WITHOUT all your layers (different filename). That way your end product isn't taking up a few dozen layers and cluttering things up, but you can always go back to a previous version to re-edit the individual parts as needed.
Look at my p-47D25 for example. I redid Rabbit from scratch this way.
I can't find a decent screenshot of it on short notice, but this is the one:
