is it paint, or is it anodizing?
I dont know how they do it on dirtbikes, but I've polished so much aluminum on streetbikes that I could puke, and I know if its anodizing its probably going to be tough to remove it from the cases...
if it isnt painted, most alumin. that comes on motorcycles is anodized or in some cases powdercoated, even though it looks like bare alumin., clear, or other color, it still has some kind of protective coating on it...
I've tried stripping anodizing with everything from various different paint strippers, to scraping it off in a mad fury with a flathead screwdriver, and the only thing I've found worth the time and mess to remove it is just good old hand sanding, or sand/bead blasting... and powdercoating is harder still...
paint stripper is nasty stuff, it goes alot of places you dont want it to go, and eats alot of things that you dont want it too... and I dont know if i would be getting it on the gaskets either, and especially not any rubber seals...
if you want it to look good and your going for the brushed alumin. look you may try to find someone with a fine media sandblaster, or better yet, a bead blaster, but I know form experience that without some kind of polished finish or coating on it that bare, raw alumin. will stain from anything it touches and is hard to get clean without washing it with some form of a harsh chemical, so you'd definitely have to paint it, reanodize it, or something afterward...