I would do a test spray on the craft foam with the spray paint, because it may be more acidic and eat through it.
Depending on how thick you want the scrollwork to be, you could use metallic paint pens (which is what I used on this:
[link]). To be honest, a good sharpie would work too after the initial coat of paint (to be sure it wouldn't bleed), so long as you don't press too hard and dent the craft foam.
If you're going to do any sort of helm, invest in one of the styrofoam heads you can get at a lot of beauty supply stores. It's perfect to stick pins into to put the pieces together, and you can mold the craft foam on top of it without using your own head.