For Battle Clash, I would keep it simple and use the cross hairs from the box art. Big Brain Academy I would use a motarboard cap, and Brain Age the head silhouette from the North American box art. For Barker Bill, maybe use his top hat, there really isn't much unique to that game.
Are you sure the cross hair is distinctive enough? I mean there must be a million games that have that kind of cross hair.
For Big Brain Academy I'm using the sketch of Dr. Lobe from the original game.
Brain Age I want to refrain from using something that was on American box art. I want these icons to be relevant across all regions.
Honestly I don't think the crosshairs are that iconic, but Battle Clash doesn't really have any distinctive emblems or power ups outside of the SuperScope, and that's best left for SuperScope 6. Although I guess a lot of human chiefs seems to have a gold flower on their outfits, so I guess that could also work.