I'm not sure of what grade sandpaper would be ideal as I haven't tried that with powder coating, but I'd start out with something fairly harsh, like 80 grit, just to get the bulk of it off. As soon as metal starts to show, I'd switch to 150, then 300 and several finer grades, depending on how smooth you want the final metal surface to look. You could practice on a scrap piece of aluminum to see what effect you like. Home Depot doesn't have the really fine grades above 600 or so, you'll have to go to an automotive paint store to get those but it could be worth it.
If you really want some bling, you could use some polishing compound and get things smooth and shiny. Polishing is usually done with a cloth wheel powered by a drill or big motor, but with the powder coat nearby, you'd be limited to hand polishing which would be a chore. And one polished element might look out of place in your engine compartment, resulting in a long list of things you want to polish to get it all looking nice. It can be addictive, I've heard, once you start polishing things.
Anyway, I'd use a hard sanding block, or perhaps one of the 3M flexible rubber ones, but nothing soft behind the paper to keep it up away from the powder coat. Use wet/dry paper with water, it will keep the paper from clogging so much. Be sure to only sand in one direction, I'd recommend along the length of the head cover. Tape off the rest of the head as a slip could mar the fresh surface.
